Since you have a small Pack, chances are you may have only 1 boys of a particular rank. In that case, a den leader cannot meet with him alone unless it's his parent. So you will probably have to meet as a group more often.
I suggest putting together a busy calendar of events to take part in. Planned well and executed well, these events will be a lot of fun and the boys will talk to their friends. they'll get two of their friends and they'll get two of theirs. You get the idea.
A fun Pack will grow. there is no doubt about it.
You can also check with local churches in your community who do not have a Pack chartered with them and ask them to drop off some literature to be passed out during sunday school or youth programs.
Good Luck. And remember, if you're one of the five dads in the Pack, It will be partially up to you to get things going in the right direction. You do that, your son will have fun, too.