I am doing some reading on this at the moment actually!
Firstly, he is 3.5 and will prefer to be by himself mainly anyway, and will prefer solitary play rather than cooperative or social, when he gets a bit older (probably 4-5 years) he should be mingling a little bit more.
Secondly, believe the teacher. It's so frustrating when we try to tell parents the truth to encourage them and they don't believe us! If she says he is doing better later on in the day, believe her, or make a meeting with her to discuss this further.
Thirdly, and finally, invite some "play mates" over for a play date! That way you get to meet another parent, and can watch your son with his friend to see what happens.
But don't be worried, that's the main thing, this is totally normal.