Ground work, ground work, ground work! Even in the wet, you can do plenty with him.
When my mare was retired due to age/health, our favourite thing to do was simply go for "walkies" on the lead rope. That's actually an awesome desensitizing exercise, because of all the random stuff you can introduce your new horse to.
That's how I desensitized Misty to motorbikes, ride on mowers, farm equipment, big metal drums, piles of random things that you just have lying around on your property... You can get your horse used to all these new things, so that when you start riding you don't need to worry about him spooking at them!
And general desensitization too, like introducing him to tarps, walking through puddles (amazing how many horses will prance around them instead of go through), plastic bags, just all sorts of stuff you don't expect them to be afraid of.
Desensitizing is a great way to bond, because when you're the one teaching your horse not to be afraid, he's likely to realise that you're not trying to hurt him and he can trust you.
Satin Reign aka "Misty"... my life, my love, my everything.