"I don’t know when the right time is,will it be obvious?"
Maybe. If you're lucky. Not necessarily. Some times happen slowly, some things happen fast.
"Obviously I don’t want him to suffer."
No guarantees- see above
" Don’t want to PTS to soon either."
There's no such thing, really, from an animal's point of view. They don't feel cheated out of extra time.
I'm not saying you should euth now, necessarily. But I have seen a big growth in recent years in owners saying , "I don't want to euth too soon" when their animal is teetering on the brink. And then being shocked and distraught if the pet does suddenly decompensate or throw a clot or something and collapse in distress.
Sorry, I'm just feeling a bit crap tonight. Yesterday I euthed two cats, one presented as emergency euth, collapsed and moribund having lost exactly half its bodyweight- I can almost always pick up a vein on a 25G needle, couldn't even do that, poor cat was already agonal gasping so I turned him over and gave the pento straight into the heart - which would normally be painful but he was so nearly dead he didn't notice. Just charged those clients the crem not the euth.
Other cat had lost a kilo in a few weeks and had a squamous cell carcinoma consuming nearly half of its tongue.
Also this week- failed to talk an owner into euth of a cat with very chronic, very severe (and painful) bilateral ear disease. Needs TECABO both sides...but one side is too far gone for that, the facial nerve is buried in a huge morass of chronically scarred infected tissue that no antibiotic in the world is going to penetrate and from which that nerve would never emerge safely of an attempt was made to resect all the crap. That one went home after another 40 minute consultation, with various painkillers, to "consider euthanasia", again.
Also this week- failed to convince a farmer to euthanase his cat, to which he was very attached. Cat was very thin, pale gums, huuuuge mesenteric lymph nodes, palpable small intestinal mass, appetite declining then not eaten in couple of days. Wouldn't euth, wanted to try something, I gave it a stiff dose of dex and said if any improvement call in tomorrow to collect pred. Cat presented the next day for euth having declined rapidly overnight.
I know these situations are not your situations. But just...yeah. I'm tired.