Ditto the CIPA answers.
Also keep in mind that "pain" is merely a signal sent to your brain. Somewhere between the point of contact (hand on stove) - to the neurons, to the nervous system, up to the brain and then interpreted as "pain" - that signal is mishandled by the body.
An analogy might by dyslexia. Somewhere between seeing the word and interpretation by the brain, it gets reversed. (I know this is a horrible analogy, but it was the first to come to mind. No offense to those with/those with children who have this condition. Several family members of mine do as well).
I can attest personally to having observed the effects of CIPA. Which may explain why your boyfriend describes it as "Liking pain".
While yes, there is a fine line b/n pain and pleasure, there are those that like that feeling and they are masochists.
The person I witnessed with this condition described her desire for stimulation that many would perceive as painful as actually a desire to feel.
Like a weak radio signal, her brain would only hear a very faint signal of pain. Thus in her desire to experience that faint signal, and the chemical changes it causes in the brain - she would seek an action that was pain causing in the extreme.
How do you figure it out? Ice water. Google Mythbusters and pain threshold. They had a show on it a couple years ago.
Have BF put hand up to forearm in a big bowl of ice water. If he makes it 3 minutes or over and still be able to carry on a normal conversation - pretty good indicator.
But please look up the mythbusters show, they had a warning of a top time limit allowed before actual physical damage could occur. I want to say 5 min, but it may have been more.
ETA - another test might be to ask him to show you a scar from a burn. Those with CIPA rarely make it through childhood without a scar of some type, and sadly it is commonly a burn - such as from actually touching the stove because they didn't feel the pain warning to remove their hand.