That's consistent with my experience as well. If you're comparing him to the kids at preschool/daycare, if he's only been there a month he's going to have a hard time fitting in right away - that's normal. Nonetheless, my son was at daycare since 6 weeks old and still didn't play with others much at two.
Two year olds - and even many three year olds - primarily do what's called "parallel play" when they do play together, which means they both play cars in the same location, occasionally interacting, but primarily ignoring the other child except when necessary.
What you can do to encourage your son to interact more is to show him how to do so. Play with him as if you were another two year old. Give him tips for how others can be included. For example, my son today (35 months old) was playing in a play area and had some boxes of 'food' he was pretending to ring up at a grocery store. A four-or-so year old came by, and started to pick up boxes also. I suggested my son go behind the 'register' and pretend to be a worker. He did so and had fun (for a little while).
A lot of what play at that age is, is learning these kinds of things. You can let it develop naturally, or you can give a few pointers to help him out - both are fine. Give him opportunities and he'll figure things out when he needs to. As he gets closer to 3, he'll start playing more with others, and probably around 3 he'll start asking to play with kids at home (though that exact time varies, from my experience).
As far as vocabulary, fifty words sounds reasonable. At two there's still a huge variety of vocabulary levels; my oldest probably had around 50 words at two, and I worried he was a bit behind, but by 30 months he was talking up a storm and by 35 months he's composing detailed stories. I know a two year old who has no language almost at all - maybe ten words. I also know two year olds who were fully vocal. It seems to vary highly at that age, but by three mostly settles out (though we have a few people on the site who are speech experts, I believe, so they can tell you more accurately).
If he's a first child, and especially a mostly alone first child (not at daycare until recently?) then he's going to be less vocal. First children are usually behind second/later children because they don't have older siblings to talk to them, and if he wasn't often with other children he'll be limited as well. None of these things are problems - the mental development is still occurring, it's mostly just the lack of opportunities for seeing and hearing others talk. Adults are further away (harder to see the mouth movements), tend to talk faster than other similar age kids, and don't talk to them nearly as often as other maybe slightly older kids might, so they don't get as much a chance to learn (or don't need to talk as much - adults are good at anticipating needs, other kids not so much).
