Keep your eyes open for developmental delays as well, such as autism spectrum disorders or ADHD. Sometimes delays in knowing how to use toys or not playing with peers can be a red flag at that age for autism. Watch for lack of eye contact and lack of reciprocal communication with peers, and repetitive behaviours/OCD type behaviours. Also, children with ADHD have the tendency to whip through the toys/learning areas quickly as well because they can't focus, as well as being quite impulsive and continually taking others things without thought for the implications of the behaviour. Both types of kids do better with structured schedules versus free play, and both quite often have difficulty with social situations like sharing/turn taking.

If the rest of his development is where it should be, then perhaps he could benefit from his parents taking him to early years programs or extra curricular activities to find peers his own age to socialize appropriately with. How will he manage when he gets to kindergarten shortly?