hi Kel84,
I am not sure if I can really help but I could give you the other side perspective since I have this addiction and I am fighting it for 4 years.
firstly it depends on his personality, I could be ashamed of my father or mother but I could expose myself to my sister if she will not tell my parent. it may be the same case with him. try to speak with him one to one, promise him you will not expose him to your parents.
tell him you are worried about him and you need to help him whatever the issue is, indicate that you will not judge him and you just need to help him to find a solution for his issue. (and if he exposed himself to you then keep your promise and don’t expose him keep it between both of you. it is enough, you can handle it together, no need for everyone to know).
the gambler feels a lot of guilt and shame and some will prefer to die and not to expose themselves. till now I didn’t expose myself to my family but if my sister approached me in this way I will.
try more than one time the first open the subject in general without mentioning gambling, the next creat a story about a where the result was the person has a gambling addiction and you are doubting the same is going on with him. I guess maximum by the 3rd time he will expose himself (if this happened to me for sure I will).
secondly are you sure it is gambling? it may be another type of addiction, either way, be supportive and don’t judge him. don’t discuss what has been lost discuss the way from now on, what has been lost will not be recovered by gambling, and keeping the regrets about what has been lost will trigger the gambling.
thirdly, he may not know that he is a gambler, I have been day trading forex markets for two years before I knew I am a compulsive gambler, I was ashamed to ask anyone so I sent an email with what was going with me to an online therapist and the reply came that I have compulsive gambling, frankly, I was shocked I thought I was trading for two years and it turns out I was gambling.
there is a lot of unknowns here, you need to try with him to know how to help him.
now from the other perspective, you are doing well by hiding the money from him, he should not has access to extra money the money is the drug in gambling addiction.
there is a lot of work to be done but firstly, you need to get him to expose himself to you.