No, I don't think tapeworms could cause open sores. I hope not, that would be pretty strange. That's the part I'm so puzzled about. He must have some kind of bacteria causing those sores. Or external parasites and he just isn't flashing or darting. But that seems unlikely.
Your poor puppy. That is a load of worms all right. Hard to believe one poor creature could carry so many. :(
Sounds like he's got the best treatment plan until the Maracyn comes. If his behavior changes at all, maybe up the AQ salt to 2 tsps per gallon. But hopefully he can hang in there. You're doing a great job, LittleBettas. :)