Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this problem with your boy; I know how disappointed you must be, under the circumstances.
Some male dogs do have a low libido (a more common problem than you'd think) and will benefit from Cystorelin (GnRH) injections; however, this drug would have to be given by your vet. After it is given, then you'd want to wait 90 minutes before introducing him to the female you want him to breed or if you are considering artificial insemination, then try to collect him.
Of course, male dogs with prostatatic or orthopedic issues might also have a decreased interest although the latter isn't likely if he's trying to tie.
And, he's a bit on the young side for the former issue but I just wanted to mention it to be complete.
I've also found that dogs and bitches who live together often require artificial insemination. In most of these cases the bitch(es) are higher in the relative order of the household and either they won't let the dog breed them or the male won't try very hard to do so. So, this may be one reason why your fella isn't interested depending on the dynamics between him and his female housemates. You'd think that natural instincts and hormones would supersede their relationship but it doesn't in every case.
I hope this helps. Deb