the bumps are NOT his testicles, the bumps are a strong muscular mass that encorges when "excited" just like the rest of the penis does. its generally nicknamed the knott and is what hold the male inside the female during the TIE in their mating.
Usually the knott will only be fully exposed when there REALY getting into it, but it is normal, and nothing to worry about. its 100% natural and nothing to worry about unless it doesnt recede within about 1/2 hour. the flesh of the penis is very sensitive and too much air exposure can dry it out if after 1/2 hour its still not receded you can apply a little vasaleen then give it about 15 mins of calm time, if after that hes still fully exposed its time to go to the vet as he could get a bactrerial infection.
but otherwise its 100% normal, one of the quirks of having a boy and YES does happen after a neuter, whiel the number of occurances should be reduced after a neuter, it basically means the plumming is just disconected, everything else works fine, he just cant get a female pregnant.
and welcome aboard Ruby