It sounds to me like he may have a buyer already that's ready to pay him cash money for the whole thing and wants you out because of that.
But don't worry. Once you signed a legal contract. As long as you can make the payments up he can't force you out.
Renting to own is a whole LOT different than just a rental lease.
When you're just renting a place you didn't sign to buy it and are never gonna own the place no matter how many payments you make.
When you're renting to own, you basically signed a mortgage but instead of signing a mortgage with a bank, you've signed the mortgage with the owner of the house.
Rent to own agreements means that he would have to foreclose on you. He can't just evict you like he could with a regular tenant.
Yes you ARE his tenant until the house is paid off and must agree to do upkeep or whatever else is spelled out in the lease.
But you're a different type of tenant.
A foreclosure process has to happen which in some states can take years before he can foreclose on you.
Keep sending him or giving him the money when you can and keep all your receipts in case you need to go to court.
I'm a landlord and a type of landlord that does understand that it's possible for people to run into financial trouble without it being in there control.
Your landlord sounds like a heartless person to want to kick you out over one missed payment.