If you build his designed airplane without buying his plans, he has a claim
against you for copyright infringement. That would be his claim against
you. Once he sells you the plans, you can build an airplane. If you do
something that is contrary to his plans, he can refuse to support you - but
he has no other claim.

You are building an experimental aircraft. Experimental aircraft are
experimental.

In fact, if he communicates to the FAA that your airplane is unsafe because
it does not use the engine he has specified and the FAA acts upon that to
deny you an experimental license, you would have a legal claim against him.

Colin