Sonja, I am sure you are really shocked
. I have never heard of a male dog being able to have/hold an erection that would allow him to be able to tie. I wonder if he has a retained testicle?
I would take your darling little Maximus to the vet to be sure that he is really truly neutered. Hopefully a breeder with more experience in this area will post their thoughts as well.
We both have been around horses and I've seen geldings make some very valid attempts but I've never seen one "do the deed".