First of all, kudos to you for being versatile :) You will be a better horsewoman in the end!
Have you tried ointments for growing back the hair? Believe it or not, I use Bag Balm. It's actually made to put on the udders of cows to keep them soft and cleaner ... but it really grows the hair fast! Just apply it every night till it all grows back :) Again, it's just called Bag Balm. Here is a link to the stuff - Bag-Balm,Vermonts Original Moisturizing Ointment -10oz on eBay!
This is just on ebay
, but you can get it prety much anywhere - tack stores, feed stores, online ... you name it :)
Hope that helped!! For now? I would just use some kid-friendly Crayola washable paint as close to his color as you can, that you can jut wash off after the show :)