Uhm. First would be: You don't expect male / female dogs to
want to get their anus penetrated.
As such only do anything in this regards with him,
if he shows interest or doesn't care (probably more common than "he is requesting it regularly").
This is based on careful - his opinion and expressions respecting - checks, basically "find out if he has any interest in this sorts of things and if not, then accept this without forcing anything".
Cuddle with him, show him your affection, show him that you're interested in his sexuality. Stroke and caress his body. I would assume that you as well care about
him getting some fun time. If not in terms of copulation, then you should at least care about his interests in masturbation, if those are existent.
The more he trusts you and opens in terms of his own sexual interests, the better for understanding what he wants / accepts. ?
For possible and recommended ways of as example masturbation, copulation (dog on human, as such inverted to your question) and analysis of a male dog, there are a lot of how-to's / educational threads in the area where you posted this thread:
Guides, FAQs, How-To's, and anything educational related to animal love.
www.zoovilleforum.net
I am not sure if there are how-to's specifically linked to your question, but there should be some discussion in the M/M topics, as well.