B.B.
Toddlers that age often don't get along with kids the same age. When my twins were 1 or 2, we started doing play dates, and there was always fighting. With twins, I think they feel especially threatened by a new playmate, since they're so used to each other.
My adopted boy and girl twins, now age 4 1/2, are both very prone to aggression. We have finally found a strategy that is beginning to work for our family, by realizing that odd as it sounds, there is always a reason behind "unreasonable" behavior. Jealousy, overstimulation, food allergies, who knows? Try to tune in to his needs and make sure they are met as best you can. The other boy might also be stressed from being away from his parents.
What kids like this need is more compassion, patience (though it sometimes seems impossible), and being heard; not more stress such as time outs (we tried those, too, with zero results and only increased stress for everyone). Being heard is especially important to kids like my daughter, who want to be in control, and insist that things be done their way or they have a fit. When they act angry or mean, they are really scared or stressed.
In your situation, it's probably a good idea to keep them somewhat separate just after an incident, but then bring them together and try to encourage sharing and nice play by telling your son that the other boy can be his friend, if he includes the boy in his games. This worked well for my kids when there was trouble during play dates.
Good luck, and peace to you and your family!