Frankly depending on when the loan was made, by federal law, he had to charge you a min interest rate of 4.67% to high of 5.36% based on the published Applicable Federal Rate set by the IRS each month. Otherwise, it was considered a gift in the eyes of the IRS and you should have claimed it on your taxes (some exceptions apply). So using 5.36% as the basis, over the 15 years your debt would have been around $110k.
As others have mentioned and he could have taken that $50k and received a lot more by vesting it in something that actually paid him back at full value. Heck if he dumped it into bitcoin, in 2010 he would have about 400,000 bitcoins ($0.125/coin in october 2010) worth at its peak (2 years ago) about 8 MILLION when it was going for about $20k per coin. So yes, if he played it right your $50k he leant you and now only wants $75k instead of the roughly $110k, could have net him 8 million. Trust me, he has the raw deal here all day long.
Some other things to think about, if he charged you interest below the IRS rate, you might have also needed to claim the money as income/donation in that tax year, so you could have filled bad taxes. He also does technically carry the right to give you a 1099-C for any portion of the $110k minus your payments which he took as a loss. He could also likely get the collections agency to put this on your credit report with enough proof of debt. He could sue you in court and then now you have a judgment on your report.
What I am trying to say is there has been a huge loss in opportunity costs for him, you might have huge tax repercussions to correct back taxes and can take huge hits now, your credit can absolutely sink, many many more bad things can happen.
If I was personally in your shoes, i would take the $75k which is a huge settlement from the $100k+ and a huge loss from just his potential 8 million on the table and count it as a win and lesson learned. I would start by selling everything you can to give him a big upfront payment to show him things are different, and yea, get that $1k/mo payment going and all future bonuses and tax refunds go to him until its paid up.
If I was him, I would insist the settlement be in writing at this point because your word/honor is not worth much after dodging him.
I know this all sounds harsh, but I want you to truly understand the deal you have on the table and what opportunities he has given up in return, and how his good nature has kept you from owing another $15+k in taxes easily. If you go back with a lower over, it is the ultimate insult for sure and I say that strictly from a numbers game.