Milk worked better for me than anything else. NO fruit juices, no tomato or vegetable based juices -- all have too much acid.
Plus milk is thicker than water and is actually easier to swallow.
keep trying things until you find something that will be his "go to" item for a few weeks.

DX 12/6/10 of T3 SCC Tongue.
Surgery 1/3/11 was hemigloss & forearm free flap, midline mandibulectomy, Neck Disection-All nodes clear.
Ended rads 5/11/11. Taste buds back to about 80%. PEG removed 4/5/12, experimenting eating real food again. If I can do this, so can you !! Stay Strong.