Check building codes in your area for adding a bathroom. Most will not allow a kitchen.

Garages are not built to comfortable residential standards. They are uninsulated, cold, unvaporsealed damp, and don’t have enough electrical service, of the right kind. To get it that way takes 50-150K, architectural drawings, permits, and rebuilding more than half of the garage. If you want plumbing, you’ll be on the higher end of costs.

Get son into a job with a contractor and he’ll make more than with a college degree. Paid to learn. Then that can be his first weekend project. https://www.mikeroweworks.org/

You should be able to do it of your septic is below level of garage slab. Water will have to come from house after your well equipment. Your water piping will have to be below frost line in your area.

It will make a difference where you live. Maine or Florida? Alaska or Hawaii? North Dakota or Louisiana? Do you need to install air conditioning? Heating? Insulate water pipes?

Buy a travel trailer with a bathroom and kitchen in it. Park it in the garage.

Converting a detached garage into an apartment will almost be more work and expensive than tearing it down and building a new MIL apartment.