Yes DS2 is (was) like that. He had always been happy to play on his own since he was very little. He would concentrate on one toy or puzzle for a very long time - it was worrying. At 18 months he could do large and complicated puzzles that his brother, who was 4, couldnt (and wouldn't) do. His mind was channelled on one thing at a time, and his first word was at 2 and a half and now, he is nearly four and he is talking but not very clearly.
He also found it difficult to make friends and socialise (except his brother who could understand him).
Now, at nearly 4 (he will be 4 in April) he is much better, plays with friends, talks quite well, but he is in a special school that has a language unit for children with speech delay.
I think the only thing I can say is to stick with speech therapy and try to get the most out of the service. Try to get his attention before you speak to him, and also try to play with him at his game (if he is concentrating on something playing one thing, talk to him about what he is doing. It's like narrating his play if you want. Don't ask him too many questions, just take ten minutes a day and comment on what he is doing, focusing only on his activity with no telly or music on).
Speak in short sentences and repeate the key words.
By the way, DS2 had glue ear for a little while, and it does come and go. He's had 3 (yes 3) hearing tests because his teacher was concerned that he wasn't turning when called in nursery. She told me that one day, he was playing with a toy and she called his name 5 times on the right side, and 5 times on the left side. Only when she clapped her hand and said his name loudly he turned his head.
He is now making a lot of progress weekly but after a lot of effort.