Consider all scenarios and prepare for all possibilities.
You have been with him for 15 years and you have a child. Leaving him is easily justifiable but you may end up being alone for life. It may be harder to find a new love as a divorcee with a child. You have so much memories with him that any new guy may wonder if this new love pales in comparison. Your child will also need a male influence in his/her life as the child grows up. You may not need him financially but it does look like a lonely road ahead. So do consider divorce carefully. Will you end up punishing yourself for what he did? Were you with him when he was a poorer guy 15 years ago and will divorcing allow the other woman to enjoy the economic benefits of his career now?
Why not suggest setting up a joint account to spend for the kid and yourself? He puts money in and you spend the money to buy things for yourself and the kid. You will be draining his ability to spend on the other woman. Over time, this will show that he is supporting you and the child financially at $XXX dollars per month. If he doesn't wake up and if divorce is unavoidable, this is useful for you.
Second guessing his real intentions is like running a set of questions over and over again inside your head. Why not engage a PI to verify his story? If he is reformed, the PI finds nothing. If he does not wake up, at least you have some evidence in hand.
Sometimes it is harder to compete with a younger woman. What she does in bed is also dependent on your man's sexual ability. How hungry he is... you will know better. Could it be a situation where he is hooked because she fans his ego? Many guys will feel flattered if someone a decade younger finds us attractive.
I hope this is just a temporary blip. Do consider the consequences of divorce. Will you end up alone for the rest of your life? If so, will you consider closing one eye for the next 5-10 years? There will be a period where the younger lady wants to get married and have kids. She may leave your husband when she realises she is putting all her 'eggs'
in his basket.