One thing not mentioned is the margins his pop was making.
Dad had no debt except taxes. I'll bet his prices reflected that. Dad will expect rent and sooner or later pay off on equiptment maybe no blue sky.
Now if he were to add the legitimate costs(rent and machines) of doing business to the prices he is charging
plus his expected salary he may find that his one customer may not like the new price schedule,by-by.
Don't know maybe dad was making enough margin, but not likely.
Now for the buy out. The son is in a bad spot emotionally. What dad thinks the business is worth and what it actually worth are going to be two different things. The son needs to pay what ever its market value is no more and use that in his business plan. Also as mentioned clear up any interest his siblings may have before he even starts any negotiations. Hard to take the emotions out of it but it is only business after all.
Seen so many cases of people who seem pretty well balanced get at each others throat settling an estate.