Hi M.!
This is a tough one. If the boys were younger, my response would be one thing~ and if they were a little older, I would suggest something else......but they're 8!!!!
I'm sure you know that usually things like this happen when a child is "jealous" of another child~whether it is about toys, or the "family life", children will usually take something (even if they don't need it) to make them "wish" they had what their friend has.
I'm hoping and praying that your son's initials are on the toys. This seems to be the only way that the boys' dad will believe you, when you say his young son is a thief. I don't mean to call him a "thief" (literally).....but, to prove that his son did not find those toys in the street :o)
The easy way out....well, if your son doesn't really like this boy, then that's the BEST & easiest solution to the problem. Stop the relationship and forget about getting the toys back.
However, if he does REALLY enjoy playing with this boy, then it's not gong to be so easy :o) We don't want to ruin the fun for YOUR son if he really enjoys this boys' company, however, things will definitely have to change when he DOES visit your house.
If the boys' dad does not believe you about his son stealing the toys, then I wouldn't allow your son to go over to their home......there would be deeper reasons for the denial on the "thievery" and you wouldn't want your son to be a part of it....
How many times would I have LOVED to been the one to have taught someone else's child a lesson????? Too many times, that's for sure :o) The boy and his habits are someone else's problem, not yours. Focus on the things you are supposed to be concerned about under your own roof. Protecting your son from a "not so honest" friend is the issue here.....right?
M., I have 2 boys, and they have been to the same elementary school. Friendships have changed each year, with the exception of 1 or 2 "special friendships". My point is, your son and this boy will probably drift apart, anyway, as they get older. This whole thing should solve itself with patience. Why put anymore energy into it? Your son doesn't need to feel "your fight" about the whole issue. School will start soon and your son will be distracted.
I'm not always this "passive" about things, but when it comes to my boys' & their friendships, I've learned that it's usually the best way to handle most situations like these :o) We've been through many different situations like these.....and none of those friends are around now. My boys "figured it out" for themselves :o)
Be the "better person" and your son will learn more then he would if you were to fight it :o)
Good Luck!
:o) N.