He will get the hang of it with time and practice so give him the opportunity to learn. He needs to get the feeling of balance to stay upright and to build up muscle strength.
Until he has learnt, I don't think you can do much more than you already are. We did the same with our son, putting cushions around to protect him but occasionally he did fall elsewhere. As long as he doesn't fall heavily onto a very hard surface, it's unlikely to do much damage from sitting height. At 7 months you can't really leave him unattended for long periods anyway and I'm sure he will let you know if he gets a bump.
For you, it's good practise for getting the balance between how much to protect him and how much to give him the freedom to develop. In a few more months he'll be walking where he'll fall from higher and survive and there will be many more situations after that where you'll have to make a judgement of when to step in and when to leave him to it. This is a relatively harmless situation to try allowing him a bit of independence.