If it was clear and runny like water - than it could be that he just had a bit to drink and it came back up - if it was not water or if he is bubbling at the mouth - it's probably a respitory infection and he'll need to see a vet.
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Tosha 
"Nihil facimus sed id bene facimus"
7.33.0 Ball Python (Harry and Fluffy and gang)
1.0.0 Angolan Python (Anakin Skywalker)
0.0.1 Green Tree Python (Verdi - yeah I know but my kids love the book)
0.2.0 Feline (Pippen and Pandora)
0.0.1 Desert Tortoise (Pope John Paul aka JP )
2.2.1 Fish (1,2,3,4)
0.0.2 frogs rescued from pool skimmer