The fact that you are asking about it may mean...
1. You don't feel 100% comfortable doing it
2. You uncle may respond, even mildly, in a way that does not make you feel 100% comfortable. Whether it is by what he says or does regardless of how minor it may be.
Follow your gut feeling to answer your question. It's OK to say no. Your uncle can pay for a massage at a massage company, get someone else to give him a massage, do some stretching exercises, take some over-the-counter pain med like tylenol or find another way to relieve the soreness.
You are not responsible for making your uncle feel better after a hard day at work.
Have a Merry Christmas.