My blowup Santa needs help

First let me ask you if you know where the leak is coming from? You might need to stitch up the leak to get a better seal. You could use glue too but not hot glue or it might melt the nylon. If it's not a leak, then your fan may not be strong enough anymore... check that as well. I'm sure you already checked the fan that something isn't blocking the intake as well.

If you do find a leak, I bet a bicycle patch would do the trick. They're relatively small, and make an airtight seal.

If you can get him into water it will be easy to find the leak, just see where the air bubbles are coming form.

I agree with Chas! If there is a leak or tear, (check all the seams), you can use fabric glue for the repair. Check that the blower is running up to speed. Motor may just need to be lubricated. Use WD40. It will loosen the motor and repel moisture.