When doing this he has a frowning expression on his face – is that a sign of frustration? If so, should I assist him with my hand, or would that be a form of bestiality?
There's an old joke that goes, "Why do dogs lick their [testicles]? Because they can!" Yes, it probably is a form of pleasure for him; masturbation (in various forms) is known to occur in lots of animal species. But that "frown" you see probably isn't a sign of frustration – determination and concentration, maybe – so don't feel you have to help. (And you're right, offering a "helping hand" would be a form of bestiality.)
