I think the simethicone would get too diluted if it was put in his water, but you can try putting a dose on some greens or fruit.
Are you using a plunger type syringe? If that didn’t come with the medicine, maybe you could get that kind of syringe at a drug store or from a vet. Don’t push on it too quickly, but give a steady push on the plunger until it is empty. If you don’t see that it has dripped out of his mouth, then it must have gotten in him. As I said, he needs to get used to getting medicine, because there will be times in the future when he needs it or if he needs to be fed Critical Care. Wrap him in a burrito wrap (you can find online videos of that) and maybe you could have someone else hold him as you give the medicine. If he isn’t pressing his tummy into the floor or shifting uncomfortably, he may not have gas, but simethicone passes harmlessly out of the body and has no side effects, so it would be good to at least try it and see if it helps.