I have replied to your question in the sexuality forum, here are a couple of more pointers:
I have found that the best dildos for anal sex are a little less firm than the ones for vaginal sex. They are similar to the Domestic Partner series (not made from silicone but pvc, always use with a condom).
You can often find them at gay sex shops and you should talk to the owners of such a shop, to help with the choice. They also fit better in a strap-on because they don't have a flat stand, but rather hollowed out testicles. But he would have to test it with his strap-on first, because they are all so different and not all fit. Take him penis shopping and be prepared that you have to look for a while to find the best combination.
You can also check out the feminist sex shops and producers who have toys specifically for trans men. Make sure that you mention you are a gay couple and want it for anal sex.
There are some newer dildos that are firm in the core and softer on the outside, I've been told they are very realistic. But they are pretty expensive. The ones I saw felt also a bit too big for anal sex, but maybe that's fine for you

Some dildos can squirt, and there is semen-look-alike lubricant on the market.
You can warm the dildo with hot water or a hot water bottle before sex, which makes a big difference.
Some other pointers about you partner- the reason why he doesn't talk about these things might be that they increase his body dysphoria (I'm just guessing here, I might be wrong).
Some trans men feel that their packer or dildo is like their own cock, attached to their body like a good prosthetic, while others feel they increase a feeling of "my cock is missing, and what is this thing???". It can also be both, and switch between these two extremes, even during sex.
This can be extremely irritating, to say the least.

He might have techniques to deal with this, for example from masturbation. i.e. techniques that help him to keep up the feeling that he has a cock.
He might be completely different, though, I'm just writing this down to give you some ideas about what goes on in our heads during sex ;-)
You really have to ask him, though.
I think it's cool that you want to deal with this and get proactive about a new cock for him

I believe, if the chemistry is right between you two and you are both happy to experiment a bit, you can get this right.