Used as a noun: He is a bum-upper (or, rarely, 'bumupper')
Used as a verb: He ups bums
General sayings: He looks like such a bum-upper! He just cannot stop upping bums. What a bum-upper, look at that!

As a verb 'bum upping' conjugates simply:

Present Tense
I up bums, you up bums, he ups bums, we up bums

(Or an alternative could be: 'I enjoy upping, she enjoys upping...etc.')

Past Tense (Imperfect)
I/you/he/we upped (a bum or bums)

Past Tense (Perfect)
I/you/we have upped (a bum or bums)
He has upped (a bum or bums)