Generally? Yes. He's not a "bluntly honest" type; he's a gently honest, compassionately honest. If he thinks a situation would be made worse by being honest, he will skirt around it.
T2 - Can they tell if someone is lying?
Sometimes, but Ilya is actually the human lie detector.
T3 - Is it obvious when they lie?
Can be, but not when it's in the as-mentioned above situation. If he's upset you will get a blunt obvious lie, tho.
T4 - Have they lied about anything they regret lying about?
It's… complicated. He lied to his family the night he ran away and eloped with Julian, which is what led to years of abuse at Julian's hands. But this also led Milo into his life, and by extension Omar, and he wouldn't give up them or the friends he made along the way. Does he wish the circumstances had been better? Well, yeah, but what's done is done. If he could change it, he wouldn't- wouldn't risk losing his loved ones.
T5 - Have they told truths that were spread against their will?
Him admitting he had an ex husband and what he had done within earshot of a few gossips led to several busybodies butting into his life and treating him a little frailer than he actually is. It's also what led to his ex finding him and almost getting him killed.