If all else fails - try this.
Get him to take a mouthfull of water and hold it.
Slip the tablet inbetween his lips - he should be able to do this without swallowing the water.
Bend at the waist until he is at forty-five degrees - the water will now slosh to the front of his mouth, and the tablet will float in it.
Stand up straight and as soon as he gets to the upright position, without a pause, tell him to throw his head back and swallow - the water and tablet will both go down together, and he won't feel the tablet.
Try a few practise runs without the tablet to make sure he's got the hang of it.
This will only work if he is relaxed, so make sure you are not tense, it will transfer to your son. If it works, lavish praise and a treat is the order of the day - but don't promise the treat in advance - it adds to the pressure.
Let us know which method works - this worked for me, but I can see huge merit in Jack's idea, so do try that first.