As gmcraft said, swallowing exercises are the most effective way to maintain and strengthen a swallow. A good exercise is the effortful swallow. He needs to swallow (no liquid) but squeeze very hard with your tongue and throat muscles throughout the swallow.

Another exercise is to hold his breath, take a sip of water, swallow, and then cough hard. Do that 3 times in a row, 3 times a day. If he's not confident drinking, then he can do it without taking a sip, but he must do the rest (hold breath, swallow, cough)