My 5 month old is dribbling heaps and keeps putting his thumb in his mouth is this possibly because he getting teeth?

Yes: Yes, that's about the time infants start getting their lower front teeth. There's not much you can do, as mother nature works these things out. Do not use topical anesthetics such as orajel. If necessary, you can use a chilled (but not frozen) teething ring.

Yes: Probably the lower front two teeth; saliva is a good thing. Don't worry. If pain becomes an issue you mayd use a product like ora-jel with benzocaine.