Possibly Consitpated horse NOT SURE PLZ ANSWER?

My 16 yr old quarter horse paint in kinda thin in his flanks and there like pushed in and hes at his ideal weight. And he keeps putting his tail up but no poop and he eats his grain,gr,and hay. I cleaned his stall yesterday and came back this morning and there was no poop in his stall. he has had issues b4 with colic but he dont act colicy or sick. but he was fine yesterday his flanks are pushed in it seems like and he dont act like hes in pain. could he be consitpated and can i give him an enema. fyi im 16 and home alone right now. and i cant drive. so i need help