J.S.
Okay, first off if you can secure things like bikes and scooters with chain locks then do it. If you have a bin, get the type of bin that has a lock and is big enough to fit all of your toys. Don't put the things out there as temptation.
When he's in your area, playing with or around your children, you have every right to ask him not to cuss or to go home if he can't refrain from cussing. If you catch him trying to steal again then you let him know that taking things without permission is called stealing, and if he wants to play with any of your toys he must ask first. Let him also know that there will be times that you say "no" and that he is never allowed to play with the toys when you and your children are not around.
If you still catch him trying to take something, then gently confront him and walk him home and you tell his parents what's going on and that you're concerned that he's going to get hurt while playing with your kid's things without supervision. Let them know that you caught him trying to remove toys from your property without permission. My point here is NOT to use inflammatory language because even though he's dropping the F-bomb and he's trying to play with toys that don't belong to him, he's still just a very little boy. And this is more of a parenting issue with his parents than with the boy.