Sounds to me like you are a wonderful mum and well done for reaching out for reassurance. If your son is understanding everything and lots of eye contact and play and the words you describe, he sounds like a wonderfully happy, communicative little chap.
Two experiences I can share ... my son didnt speak much at nursery but understood everything and the nursery called me in and said, "He doesn't say much" and I said, "But he understands everything and Einstein didnt speak until he was 7" I know, I know over protective mum. But when he did speak, for a while only me and his big sister could understand and then all of a sudden it snapped into place and he's extremely witty and all the rest. At one point he did have a little stammer because big sister kept speaking for him and it was the frustration, but when we asked her not to and also made sure well-meaning grandparents didnt say things like "Spit it out" that ceased.
I guess a little bit worried by the nursery, I made a big thing when shopping to name the different fruits and vegetables together and a lot of bedtime cuddly toys had v unusual and tricky to pronounce names and when we did the thing in the car about spotting a yellow car we had to shout out Spliddidysquadwidge ... as I thought it tricky to say!