I am in the Northampton area and I suspect i am not the only dad having this problem.
Since separation I have time with my son alone, he is 4 years old and very active. Nowadays it is up to me to entertain him during these times and I am finding that most of his social circle (which is good by the way) is linked, via school etc, with my wife and not directly with me. It was OK when we were a team, now I have to catch up.
If there is anybody in the area and in a similar situation reading this, with children of a not too different age, then would they like to get in touch with a view to maybe meeting up at a childs play area or similar. It would be a chance for us to compare notes while our kids have a good time and make new friends.
By the way, not meaning to be prejudicial, would not limit this to fathers but any parent who would like to expand their child's social circle. Mine is very gregarious.
This isn't much use to me as my children are the wrong ages and I don't live near, but I did want to say what a good idea this is. Being a non-resident parent can be hard work as you have to entertain your child away from their main home.
If you don't get anywhere here it might be worth trying to see if there is a local Gingerbread group for single parents and if they include nrps. That could be useful.
The other thing you could try is getting a good variety of toys and books where you live so that you and your son have things to play with at your place. Car boot sales are a good place to do this cheaply. Actually, young children often like boot sales as they can buy loads of junk for not much money (you could give him a pound or two to spend himself on anything he wants).