May I ask how your vet did this myco test? Considering that ALL pet rats have myco you may have paid through the nose for a test you should never have needed.
There are a few specially bred lab rats that are myco-free but they are few and far between, and very expensive to buy. The rest of us have rats with myco, but some rats have weaker immune systems and when the myco is constantly bombarding their immune system, some stressor allows a secondary infection to come in and take hold.
You CAN test for mycoplasmosis but it takes a very dangerous blood sample, sending it off to one of a few special labs to do a serology test on.
Please keep it in mind "vet" doesn't always equal "good vet".
I have a vet up the road from me, who charges over $1000 for a tumour removal, $275.00 for a euthanasia, and now that I know better (I was a newbie at the time) and can double check their "services" with others, they don't medicate the rats properly at all.
I have a great vet now, but I had to learn painfully and expensively the little mantra above. Some are useless and can practically kill your rat with the wrong meds, overdose,etc. Some are very expensive and money-grubbing..,so you have to always double check with people about your treatments. Antibiotics are very specific to certain issues, there are some good broad spectrum ones that are often used, but some vets don't...
Sooo now that we know your rat is sick, lets find out what his symptoms are, and what your vet has given you to treat him with.
Once he's not sick again, you should get him a friend. Rats are very social creatures and just aren't their happiest alone, no matter how much time you spend with them