Is my horse underweight?

I have a 14 yr old quarter horse paint gelding. He is VERY stocky and his muscles are huge. Hes also 15 hands high but you can see his ribs a little bit. I'm assuming he has worms but he's in very good health for the most part. I've used safeguard but it's done nothing for him. Can I use Quest dewormer? Becuase it says you shouldn't use it on underweight horses