If he is actually engaging in group play 25% of the time before age 3, I don't think this is that bad. I think boys especially take a little longer to get interested in playing WITH other children. Which is purely anecdotal, based on having a son and a daughter and watching kids at preschool for 4 years and how social relationships play out.
I would continue to watch it and maybe talk to your pediatrician. If it is a preference thing, then I wouldn't worry too much. If he seems UNABLE to engage with other kids/people over the next 6-9 months, that may be another matter.
