My cat has UTI crystals. Orbax isn't working. Can I add Clavamox?

Aw, handsome boy!
I'm sorry he's having some issues. While I wouldn't say its dangerous to give clavamox and orbax together, I don't recommend that you do that without talking to your vet about doing a urine culture, to identify for certain what bacteria are present and what medications will kill the bacteria. Treating without knowing whether the additional antibiotic is necessary could lead to bacteria becoming resistant to those medications.
Ensure that he IS ABLE to urinate and pass urine, as male cats tend to have very small urethras and could get an obstruction in their urethras from inflammation of the tissue and crystalline and mucoid debris in the urine. Occasionally, stones may form in the bladder and can also cause obstruction in the urethra. If your vet didn't do an Xray, you may want to talk to your vet about doing that if the problem persists.
For now, if he IS DEFINITELY passing urine, he's eating, and he seems to be feeling ok (not vomiting, etc), ensure he's getting a lot of water to drink--you can switch him to wet food to increase his water intake, and consider a pet water fountain. Keep multiple sources of water available throughout the house as well. If increased water does not help reduce his symptoms after a few more days, take him back to the vet for further workup (possible X-rays, possible urine culture).
Thanks for asking Petco Pet Education Center, formerly Petcoach!