Mostly all he's getting "extra" out of the puppy food is higher protein levels. One could switch to performance line foods and get that, plus there are adult formulas which have as high of protein levels as do some of the puppy foods. The question I see pertinent here is, which is going to be easier for Dale to eat size of kibble wise? More the case of what happens after one has their tonsils out sort of - soft foods and liquids for a while. If he's doing fine on on the 1/2 and 1/2, then switching him totally over to the adult I don't think is going to be a big deal - he'll be getting everything he needs nutriton wise for his recovery. You're already moistening it, it may have to be soaked for the first few days after the surgery, but then the puppy kibble might need to be too.
To help him not lose weight or go off his food, I'd look at supplementing whichever kibble you decide on, with extra goodies to keep his strength up for the recovery. Frozen Bil-Jac, good quality canned, Nature's Balance rolled food, fix him eggs, boil hamburger for him (or feed raw), boil some liver and give him small portions of that, canned salmon or mackeral, cottage cheese, grated cheese - go with things like that. Help keep his appetite up and easy for him to eat.
As for the kibble, you aren't going to be "wrong" whichever way you decide to go; whichever one your comfort level tells you is right - then that's the way to go.
Debbie