My nearly 3 ds has always been one to play alone, lots of role play with toys and gets engrossed. Has a range he likes from pretending to put out fires with playmobile to drawing, football etc. He loves playing with his dad me too, dotes on his baby sister and has 2-3 special friends who he has known for a long time and will play chase games etc with for long periods.
Nursery (he goes 2 days a week and been there 3 months) have approached me about the fact he likes to play alone. I confirmed he has no negative interactions/ issues. I have noticed too that at playgroups he mainly plays alone ,very happily, and rarely plays with other children. He is happy though and if another child comes over he will share toys reasonably and occasionally with 3-4 yr old boys get into games with balls. Mostly he ignores other children aside from a few special friends. He was like this at his last nursery, though there were two boys he played with sometimes.
He's been getting annoyed and clearly saying 'no' when they push him to play with others apparently so I said to leave him be if he's happy. They seem concerned and want him to interact more. IABU to say let him be? He's happy and bothers nobody. His last nursery also raised this issue. I'm not concerned, he gets loads of social contact when out and about with me and seems happy, I have no concerns about him at all. Like any toddler he's sometimes quiet, sometimes loud, loving or awkward throughout the day.
I reckon he will be better when he moves up to an older room as he likes older children and most in his room are younger. His dad and I are of a similar character.