For a gift, you can just write a check in some multiple of 18 dollars. 18 is an auspicious number. If you don’t want to write a check, gifts that are “grown up” are a good idea.
As for the invitation: Bar Mitzvahs are kind of like weddings. More people will be invited to the ceremony than to the party. The party will usually be somewhere other than the synagogue. Either way, you can definitely accompany your son. You don’t have to, though. The boys will want to spend as much time as possible by themselves, playing at being adults. You’ll probably find the service pretty boring, too.