Of course you should get a big break as the both of you will be avoiding most of the costs of running a real business.
Included in that is the extra risk you will be taking that the car will work afterward and into the future. You of course have no recourse if he fails in your eyes. No insurance against consequential damage. No guarantee against not getting your car completed in satisfactory time interval. His real job will have to come first as that relationship pays his mortgage and everything else.
Bosch Master, ASE Master, L1
33 years MB technician