So what you describe is generally possible. Though there are a lot of things you need to consider.
- Your parents raised you. What was that like for you? We all change over time, but a very important thing to remember is that your parents raised you. If you think they did a good job with you then, give them some slack.
- Your parents are not you. Your choices on how you raise your children are yours. They should respect and support them. If they can't or won't then you should not leave your kids with them. Now you need to temper this a bit. Your children will be stronger and more able to cope with real life as adults if they experience different things as children. One of those "things" come from the fact that different authority figures have different rules. So, again, give some slack, but if your parents are not respecting the way you want to raise your children in some core way, then it's time to get nasty with your parents. (read on before judging)
- Your child may be "home sick". This is very tricky. But at home, it's his toys, his room, his shoes, his house. Even if he is very sharing and friendly he is making the choice to share his toys. When he is not in his house that choice is removed. He may just be feeling those feelings.
- It could just be culture. Again you mention that you live abroad, but even if that weren't the case, different generations have a very different culture. Small tiny things could just be adding up.
Now I mentioned things to consider that more or less say "let it go", and I don't really mean that. The thing though is it's easy to consider the simple fact "son is upset, make son better" and it's harder to sit there and go "son is upset, he will have to learn to cope" but those coping skills are VERY important. So consider everything before acting.
Now with that said, one of the largest problems I have with my "parents" are that they don't respect the rules and what not set forward for our children. (We are foster parents and sometimes the rules won't make sense, but are very important.) If your parents can not follow your rules, then you need to step in and just not let them take care of the kids. Even if that means being nasty with your parents. Your concern is for your child. Your parents are adult and will get over their ruffled feathers, or not, that is not your "problem". Yes you want to maintain a relationship with your parents, but now that your the parent your first goal has to be to your child, even if that means some trouble with your parents.
That said, you really need to consider what rules your trying to impose and rather they are "good rules" or just rules for the sake of rules. Remember your parents raised you. You can probably trust them. In my experience, 70% of the problems like these are just communication. You can get around them with a simple conversation. "Hey, son is kinda sad when ever I talk with him. What's going on?" is a great way to start. That other 30% can usually be addressed with "growing a pair". Something along the lines of "I know your my Mom, but this is my son. I said don't do xyz. Don't do it. What would you do if you though someone was hurting me? Realize I will do the same and I do take xyz that seriously."
Unless your parents are just bad people, once they understand what your parental wishes are, they will likely not have a problem with it. In fact they will respect you for it. But you do have to let them know. Your can't expect them to know that xyz is wrong, just because you think it's wrong. It doesn't work that way.
If your parents really are bad people, and just disregard your wishes, then it's time to cut them off.
Now every parent has to go though this at one time or another. "Don't feed Billy candy." They take Billy out and feed him candy, and when he comes home you notice and you ask, They say "It was only a little bit. It's no big deal." Then you have to decide. Do you try talking to your parents again, do you ban them from taking Billy out at night, do you ban them from ever taking Billy out alone? It depends. If Billy has diabetes then maybe it's time to go to the outright ban. Otherwise it may just be time to "ground" your parents for a while.
The point is this. It's easy to always try to protect your kids. One of the hardest things to do is let your kids get hurt. To sit there on the side, and watch as your child does something that you know will hurt them and to have to decide to let them make the mistake get hurt. It's very hard to do. It's also very important. Yes this experience may be hard for your son, but in the same way you have to let your son make his own mistakes you have to let him experience these unpleasant things. Just like you may let your kids make small mistakes where you know they will get hurt so that they learn, you also limit that by making sure that getting hurt is not "that bad". In that same way if your son is safe, and no real harm is coming to him, then let him experience what saying with your parents is like.
However if your wishes are not being followed and your son is actually getting hurt, or something is happening that you think is bad, then go full out protect mode. Your parents will eventually understand, or not, at that point it's not your primary concern.