Save an empty dewormer tube or use a 60cc syringe with tip cut off.
See if using a “familiar, not scary” dosing device plus yummy applesauce in it will change his mind.
Practice several times with NO Dorm in dosing device, maybe finding something that he really likes and will gladly reach for the dosing device to get. Applesauce, fruit baby food, frosting from a can, whatever horse might like. When he is eager to get the dosing device, layer in the Dorm.
Put some applesauce/whatever in dose syringe, add the dose of Dorm gel, top off with applesauce/whatever.
Check with your vet about doing a trial dose of Dorm.
Sometimes we just have to out-think them.