I know it's yucky stuff
, but tick borne illness is no fun, and the rate at which he is picking them up leaves him wid open to it.
You could try Revolution (although I wouldn't trust it for heartworms).
Preventix collar for walks (but I know so many people are scared to use things like this - it would be something to ask the vet about though).
Or permethrin... If they can tolerate it..
If you're already spraying the yard, then I just don't know.
Have you tried walking them on a paved trail?