Some ideas: Ways that you can try to gravitate into making him more comfortable to get you involved.
1. At the right time (like during intimacy or you'd know better when it's right), suggest or offer him the option of roleplay, letting him choose roles & scenarios. Go along with whatever he chooses, even if it's not medfet. Getting used to incorporating roleplay into your lives can lead the way to medfet eventually being roleplay.
2. Suggest that you want to take turns playing doctor on each other, as it appeals to YOU. If you make it about "your" medfet, he may learn to incorporate his desires into the relationship.
3. Follow the advice of the .38 Special song Hold On Loosely. He may feel the need to use online activity as an escape if he feels smothered.
4. Try to be open to giving him as much sex as he wants. If he is satisfied to a worn out status, he will have neither the desire nor the energy to cheat.
5. Avoid sounding judgmental on any issue. If he feels you are judgmental on many things, he will never want to open up to you, for fear of being judged. Which is why we like this website community so much, because people are open with their fetishes and for the most part, don't judge others on theirs. Him finding a similar online community is actually healthy & a step in the right direction.
One of the reasons time is a healer for folks who are not ready to step out of their fetish closet is they often think they are alone, & sometimes they are afraid of being mislabeled because the outside world does not understand nor relate to fetishes as normal in a public sense. A lot of what you rea discussed here is never discussed with people in the outside world, where folks are judgmental & keep their own fetishes hidden safely away.
Again, patience & time is the key, And because you're so understanding & supportive & want to be involved, eventually your relationship should thrive.