Gogo boys are going to assume you want sex but will be quite happy if it's only a massage. As a447 says there's no guarantee they'll be any good at it. It will be an expensive evening however - drink, off fee to the bar and payment to the boy, all for a massage that could cost 250 - 500 baht depending on the length of the massage and the premises themselves if you just go to a massage place on Silom or Suriwong - assuming that the look of the boy is important but the massage is just that - a massage
If it's a massage + hanky panky then maybe somewhere like Apsara which is one of those places near the BTS station at Sala Daeng - but they tend to tell you which boy is next rostered on. There's also a twnky boys massage shop across the road - can't remember what it's called - but if you loiter on the pavement just down from Silom Complex (down means toward the park) you can't miss it and they'll have clocked you before you see them! There's also some massage boys in the street back near Soi 4 (Balcony/Telephone Bar) outside KFC and there's also a massage shop inside Soi 4 right next to Connections Bar where I'm pretty sure the boys are "accommodating"
Yes, just get on with it and go as you're not getting any younger whilst on here asking for tips when the place ( Bkk) is coming down with the guys you say you seek. As already mentioned Soi Twlight and the guys sitting outside Bonny massage might be a reasonable place to start if you're unsure.
When are you visiting.....just in case it makes a difference ? Or maybe you're there already, if so perhaps try out a few of the suggestions above and let us know how you got on with a quick trip report "eh"?
There is a "sticky" or pinned thread at the beginning of this forum that discusses many Bangkok massage shops. But even the author says that if you want twinks the number one recommendation is " go to Pattaya".
I'm betting I know the next question.......