My DS is just 3, but he can't seem to concentrate or focus unless doing his own thing.
For example I take him gymnastics and he is so happy when it's his turn and comes back beaming 
. However he cannot listen to the coach speak to the group, he wanders off. The coach is great very engaging, jokes with the kids etc. DS then doesn't know what to do on his turn. He starts playing imagery games by himself. The class is 2.5 to 3 years. So there are younger ones and he is the only one like this.
DS is the same at preschool, won't sit for story time, doesn't talk to other children. However he role plays like doctors, fireman, tea party. He will also small world games with figures or cars with loads of sounds and some, but limited speech. He will interact with other children at times, but won't talk with them. He seems to be in his own dreamworld.
DS has had a recent hearing test at preschools request and can hear well.
Is the imagery play with no props a time of stimming ? It's hard to explain he can act out certain things like driving a car. He has a a number of different ones he does. They look quite good, but I don't see others doing this. He does it when bored, I think. He doesn't suffer meltdowns etc.
Preschool say his speech is delayed as he won't talk there, he does talk at home, but he's not a chatter.