I've lived in my house for 3 months and have had many problems with drains, toilet and mice. Can I leave?

Complain to the landlord and demand that these things be repaired IMMEDIATELY! If not, let him know that YOU will get the repairs done and take it off your rent for the next month! It's ridiculous that he blamed you for having boxes. That's a dumb excuse because he doesn't want to be accountable and make the repairs. Sounds like he's a slumlord. Again, if he doesn't do the repairs within 2 weeks (AT THE MOST), get them done, get receipts and take it all off the rent.