IME that is sometimes a symptom of stress and ulcers.
Beyond that, I'd suggest doing what Kate says -- absolutely never hay him when he's been pawing, just turn on your heel and take the hay back to the barn. The first few days this may get rather old, but he WILL catch on and it is a highly worthwhile lesson to teach.
Sometimes they do that when they've had to wait too long to eat -- feeding more frequently, or free-choice if feasible, can often help.
If you don't want him digging a hole under his water bucket, put a mat in, even just a single 4x6 section. Some people will tell you to put a few large boulders where he paws -- IME this is a bad idea as leading to injuries and not actually stopping pawing anyhow.
BTW, not clear on whether "eats" includes grain, but if it does, being2 1/2 and part draft i.e. predisposed to joint and metabolic problems, he would be
much much better off
without any grain (nor pellets) at all. Just give him more, or better hay. Of course if he's only getting hay now, just ignore the foregoing
Good luck,
Pat