Update: I've been going to this guy now several times a year. We've become friends. The massage is always the same, fully nude with draping over the private parts. I always get an erection and my cock often peaks through the draping. He often adjusts the towel to cover it and other times the towel falls as he raises my legs to work on the sides. He adjusts it but not right away.
Last time was different. I arrived, we chatted as usual and then I went to the room to undress. He came in as I was taking my underwear off which has happened before so it's not a big deal. We chatted as I took my place on the massage table naked, face down. Normally he places the towel and starts the massage but to my surprise, he just started the massage without any covering. He didn't say anything at all, just started the massage as usual. He did my full back and legs, sometimes grazing very close to my butthole and continued very professionally.
I was not expecting it but it was hot. I thought he'll probably put a towel over my cock when I turn around but nope. I was lying fully naked with a boner as he was massaging the front. He touched my balls sometimes as he was doing the legs and ran his hand over my stomach under my big erect cock and continued the massage as usual. Nothing sexual.
It was definitely hot but also kinda awkward because we are friends at this point. At the end of the massage we didn't say anything at all. I walked to the shower, dressed, we chatted a bit and then I left.