What you want is a legally valid and enforceable waiver of liability, which may be possible to an extent, but not necessarily under all circumstances. Drafting such documents properly requires an experienced lawyer if you want it done properly. The nature of this site is not to provide free legal drafting services , especially in areas involving significant liability exposure. You should consult with an experienced lawyer, who can better advise you after exploring all the issues and deliver a properly drafted liability waiver. In the meantime, you should require your son to incorporate or form an LLC, have all contracts with clients signed with that entity, and have the company purchase a comprehensive commercial liability policy providing at least $1,000,000 coverage. Such policies are relatively inexpensive and well worth the cost. If you own your home, then you should contact your homeowner's and auto insurer and inquire about purchasing an umbrella policy of $1,000,000 or more for your personal liability. Umbrella coverage only costs a couple hundred dollars per year, and is well worth it.
This is not the proper forum to give you this kind of advice. You basically want us drafting a contract with you. But there are many other issues involved like whether your son is allowed to do this in your home, whether you are going to be jointly liable for any injuries of his clients, whether obtaining insurance for third party injuries, or forming a company.....I would strongly encourage you to hire an attorney in your area. Best
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