re: Looking to buy a coworkers house. Loan assumption no longer permissible?

You can pay him the cash and just get a mortgage/loan for 20k to pay off his note.

You can buy subject to existing mortgage and just not tell lender...they typically have a clause which will allow them to accelerate the loan if they find out and choose to do so. This method does put seller at risk of you defaulting or paying late, which would damage his credit, not yours. That seems unlikely given info provided but still possible.

Closing is kinda expensive. A basic loan for 20k would likely be less costly.

Odd to hear of someone with 50k cash buying a first house...for 70k total. Congrats. Dunno if you are a miser, frugal, easily pleased, or something else altogether but financially responsible has to be recognized.