Who is your mortgage with? Most CDN variable rates charge a three month interest charge, not the standard interest differential. Might be less than $7500.
Consider if you sold today how much the place is worth to you (ie, how much in your pocket). If ~ $100K (your $50K plus his $50K) is roughly 20% of the value then the house is worth, $500,000? If you sold using a realtor you're going to pay at least $15K in commissions. So if you sell, he's out $7500 anyway.
Maybe it's worth the refinance charge for him to get his money. Is it worth all or part of it to you to get him off the title?