Likely all you have to do is remain patient and let him adapt. The situation is quite new and he needs some time to acclimate. These days, with so many children in pre-school settings at an early age, sometimes too much is expected re: socialaizing and children's readiness to do so. The same applies to toilet training. Three weeks is hardly any time at all. Adopt a supportive, reassuring, very pateint approach, rewarding him for successful use of the potty. Take advantage of the many useful books and videos about the toilet training process. Such resources help children develop motivation to engage in the process. Don't be at all alarmed if the process takes a matter of months. That would not be at all unusual.
Each of us is unique. Some are extroverts and some introverts. Some are very sociable and some tend to be loners. Neither is abnormal, although our current culture favors the outgoing person, and tends to see him as the norm. It isn't, nor should it be. If your son is happy as he is, leave him alone.
Toilet training usually takes longer than three weeks for a boy. So, don't worry. Regarding his social skills, if he is talking, make sure that he is exposed to lots of social type activities with other kids of his age: playdates at your home, going to the play park, little singing or dancing class. Good luck.
Thanks very much for your quick reply. I think you're right, of course, but it's very reassuring to hear! I appreciate your help.