IMO and experience, your son has a 80% chance of regretting a real estate partnership. That’s about my batting average of the dozen I have been in. But, the one home run I did hit was worth regretting the bottom 80%.
We need more detail on the deal.
What might make this a great deal. Not being on the mortgage. A great mortgage rate. Good cashflow. Well maintained property. Buying at a discount without real estate commissions and closing costs. Having the resources and liquidity to buy out partner A at anytime.
Partnerships can be a form of asset protection for bankruptcy and or divorce.
If the deal somehow requires getting on a mortgage then I say no deal.