Set your partner to aggressive, and every now and again he will fall down and call for healing. Just heal him 25 times, it doesn't matter if you drag him or not, it is more of a suggestion seeing as his initial spot will likely be under enemy fire.
It can be done by shooting your partner yourself, but this can be a tremendous waste of ammunition.
It is probably easier to find some good cover on the highest level of difficulty, and set your partner to aggressive on your position. That way he will pop his head out to be shot, but stay near you.
If your cover serves well, he will be downed at cover, and healing him will be simple and without delay.
Alternatively, you could always have a co-op partner run out and get downed on an easier setting, so that you can heal him, rinse and repeat.