I would not worry about one episode of regurgitation. The milk replacer may have upset his stomach. Give him something bland today (plain, onion-free meat baby food mixed with baby rice cereal is great) and try his normal diet tomorrow.
As for the meds, the rule is to NEVER double up a dose. Give the dose as soon as you remember it and resume a normal schedule. Realistically you can give two doses as little as 8 hours apart (and then go back to your normal 12 hour schedule), but I would definitely not give them any closer together than that. Unless the vet (or any doctor, actually) specifically tells you otherwise, it is much better to miss a dose than to risk an overdose.