I bought a house on contract and have some "What if's" I'd like to run by someone

So I bought a house on contract from my grandmas neighbor and basically how it works is that i pay x amount of money each month for my mortgage in a check written out to the guy who sold me the house which he then puts into his bank until the house is paid off but some of my biggest concerns are 1. What happens if I give him the money and he just decides one day to not pay it, do I just lose my house? 2. Can he randomly change how much my mortgage payments will be? 3. Before he sold me the house he was leaving it to his son on his deathbed is that overturned because I bought it? Please let me know I'll do the best I can to answer any info that I left out