I hate to give advice when I don’t personally know the person. I only know for myself and for other that I know in such a situation (whom I asked). I won’t tell you what to do, only what would have worked for me and my friends.
You said he believes in tefilla, that he says he knows when things don’t work out it’s because of your tefillot.
When I was finding my way, the more people reminded me of where I should be holding, the more I wanted to improve at a slower pace. True or not, I felt I had to prove that my worth was not dependent on my relationship with G-d. Even if I had already decided I was going to work on something, as soon as someone told me it was where I needed to be, I would get resentful and lose interest. As my friend said, “We all want to be in control and as soon as someone tries to cross that boundary, we get defensive.”
When people stopped reminding me that I had to daven or cover xyz, that was when I was willing start working on it. I had known it was true, but the more it was shoved down my throat (even respectfully) the more I felt I had to be rebellious.
If he is in the same place, he needs time and he will come around because he knows it’s true. But it needed to come from myself; perhaps he’s the same.
Nobody becomes frum because of the people who throw stones ‘in the name of Shabbat’, but many a person becomes frum because of an act of chessed. That’s why I said to make yourself enjoy it more, so he sees it’s something worth striving for.