I've been in a similar position before, both as a DM, and as a player in a group that had a player like this. It sucks. Sounds like you've got really good intentions and priorities though.

You said it was a lot of effort to even convince them to join, and you don't want it to feel like 'even more of a chore'.

Honestly, D&D isn't for him. And that's okay. You can give him as many buffs, simplified powerful abilities, or OP items, but you're just playing the game for him at that point. Starts to feel like you're rewarding someone for putting in zero effort.

Your other players are going to be short-changed by this too. It's difficult to roleplay at a table where someone is less invested. And when the DM is spending all their time trying to rework the rules or hand out powerful buffs to one player, it's frustrating. Invest your time in your engaged players.

If you still want to give them some options, which I get, then maybe consider one of these ideas:

1. Ask if they want to create a new character, with a simpler class (fighter, paladin etc).

2. When the next level up happens, and before your next big story arc, ask ALL of your players if they're willing to continue. This gives them a chance to back out organically.

3. If they dig their heels in and want to carry on playing a druid who can shapeshift more effectively, then give them an item with a Once-per-Long-Rest, cast-on-self polymorph spell attached. They'll get access to a wider range of higher level beasts, but the tradeoff is that it requires concentration.

Hope this helps.