What to do with my Leon

While not bad, those ivs are not desired, but workable. Since he is -spd you can make him into decent mixed bulkie archer who can deal with physical and magical threats. Still something like +def or +atk - res for this set is preferable. For mixed tank he can be -hp +res with miror stance, resistance +3 or distant defence 3.

I don't see why you don't just wait. Unless you are planning to merge him.

Are you planning on merging Leon into a +10? If that's the goal, then certainly you can start training the +res/-spd up to max lvl, unlock to 5*, then use all that extra SP to train him up with the expensive skill set that will make him into the "Mr. Mega-PhysDmg" tank that he has the potential to be (w/ something like Guard Bow+, Close Counter, Quick Riposte 3, etc). Then, when you get the next 13 extra Leons in your "barracks," (I assure you, he's quite common since they made him a 4* AND 3*) you can max out the one with the most suitable set of IVs & merge this one into him for a +1 Leon w) much less in the way of SP to earn before turning into a beastly bada**!

That being said, those IVs are definitely not the ideal choice to waste feathers on if your end game is just having one, 5* lvl40 Leon to use for various purposes, so if that's the plan then hold out for the very best IVs before you start training. :-)

If you're not planning to merge him, I would wait for better IVs. The best thing I can find to do with the exact unit you're presenting me there is:

Guard bow +res
Bonfire (or ignis)
Distant def
Guard
Atk smoke (or any C-skill)
QR

And make sure he gets a res buff. That will give him at least 38 res, which is quite thick for Leon. He'll get doubled by any moderately fast ranged unit (Micaiah, Julia/Deirdre, and Lilina don't really belong in that category, but sometimes you will need to be careful), but he'll wall hard with solid defenses and prevent them from hitting him with moonbows and what not using guard.

There are other units who can utilize this build better than Leon, so I would say, all other things being equal, that you are best to wait for a better IV.