I agree with Miss Lydia... Buy some dried meal worms & toss a handful in one of the containers he likes to swim in. There is no way he can resist worms in his bath. Eventually you can throw them in the bigger pool to coax him in. He might not like the kiddie pool because they're hard to get in & out of. Does your kiddie pool have pavers or ramps inside & outside to help him in & out? My duck hates kiddie pools. After going thru 4 of them, we finally bought her a few mixing tubs from home depot. What great sturdy little pools these are! They only cost about $5, hold 10gal of water, & it seems like they'll last forever. She doesn't need any steps or ramps to get in or out as the sides on these are sloped. She was afraid of it at first (because anything new to a duck is bad) but we kept tossing her dried meal worms in there & on the 3rd day it was her new favorite swimming spot. If you can get him swimming more, that should take care of his bug problem. I also agree that he needs something with a door (like a crate) to keep him safe at night. You'll have to shoo him in the first few days, but he'll adapt to that after a couple days too. You are a good person with a big heart! I'm always so grateful to hear about people who care deeply about animals in need & aren't afraid to scare off rotten little boys! 
Good luck & keep up the good work!
http://www.homedepot.com/p/Medium-Mixing-Tub-26100/300488545
Good luck & keep up the good work! http://www.homedepot.com/p/Medium-Mixing-Tub-26100/300488545
