Graze until they dry or put a fly sheet on them
I would let him roll before the bath in clean sand or hog fuel. Horses need to roll for a number of reasons, so maybe that will get it out of his system.
Graze them until they dry
After all my horses get bathed I make sure I throw them a flake of hay and keep them inside a stall so they are occupied and don't roll. Hope this helps! :)
Would you recommend tying them in the shade or the sun.
What works for me is keep them tied up until dry and if they keep moving then let them stay longer. You can try that but it's just what works for me it could be something different for you.