But, remember, first rest . You must rest until no pain. Finally, the musical public has become aware of the problems in learning an instrument and I have seen all kinds of devices to aid that right hand. A good thumb rest will help, Make sure it is padded and firm. Then there are the plastic affairs that support the thumb from wrist to thumb.
Rest is first.
Make sure you begin quite slowly, for short periods of time only. Do NOT wait for pain. Stop prior to pain. If he says yes, try it. If he is not, he may not be able to help much.