I'm no expert by any means, but I do a lot of research to ensure everything I do with my horses is done safely. My understanding is that you should never tie solid in a rope halter. Should he get caught, get a leg over the rope (even if you don't give slack, some will rear and kick out with their front leg), you have no way of getting that halter off. If you are going to tie solid and leave him to teach him "patience", use a breakaway halter.
Honestly, I'm not a fan of tying a horse and leaving him unattended. If you have to do this, spend some time teaching it first. Clicker training would be one way. Wait for the horse to stand quietly as you stand just out of sight for very brief periods. When he is quiet, you click, reappear, and treat. Don't do this when you're in a hurry, schedule tie-training sessions when you're not likely to get frustrated because it is taking too long. If the problem is that you are out of sight, you need to work on this, specifically. Leave him briefly at first, then longer and longer. My Rusty was not happy about being tied solid at first, especially when I went to get his saddle out of the tack room, but now it's just a normal thing and he stands very still. It just took time and patience. But I understand your horse might not be quite so easy get out of this habit.