Controversial but honest.
Stop shielding his dad!
"Not slating him" is NOT the same as excusing inexcusable behaviour.
My dd and I have been through similar, without the gambling.
I now very much regret doing as you did and as SO MANY single mums are advised "ahh they're too young to have to deal with the reality, don't interfere in their relationship with their dad, don't speak bad of him" blah blah blah
It does NOBODY except the deadbeat arsehole any favours!
My dd was blatantly honest with me when our situation reached a peak where I had to tell her that he wasn't paying cm properly, wasn't making the effort to see her and I could no longer afford to cover for his fuck ups and shit attitude! She was angry at him and hurt at his lack of effort but she was also angry and disappointed with me for misleading her and basically prolonging a situation that was really an illusion.
Be honest, in an age appropriate way.
"Mummy and dp make sure you get all the things you need"
"Daddy DOES have money you don't need to worry about that"
"Not everyone drives but there are buses and trains and taxis which are how people who can't/don't drive get about. Daddy is perfectly capable of doing this too"
When I stopped (partly as discussion with dd, she wanted her dad to show he could make an effort and I don't actually blame her, but partly because I plain couldn't afford to keep paying out cos he'd messed up yet again!) bending over backwards facilitating my ex's relationship with dd FAR Beyond what was reasonable he very quickly stopped bothering not only with the stuff I'd been sorting for him but even free and easy things like phone calls, greetings cards, presents at birthdays/Christmas...
He is a worthless piece of shit and my dd deserves better as does your boy, but I'm telling you the older they are when they realise their dad is a lazy, useless arse who doesn't really prioritise them, the harder it is for them to deal with.
For me that is not only based on our experience but having seen many family and friends going through similar, not just dads either but nrps even if women it really seems to be a case of "out of sight out of mind" for a lot of them.
Definitely DON'T cover for him ESPECIALLY if it leaves you out of pocket.
The thing that led to dd realising was she told me dad had promised her X thing for Christmas but didn't mention it to her dad, so when X was under the tree wrapped and marked "from dad" it all kicked off. So not the best Christmas and I very much regret that, and genuinely thought I was doing what was best for dd in the circumstances.
I now just think I was an absolute mug to cover for him for so long and allowing his shitty behaviour to cause (albeit very temporary, she understood I was trying to be kind) upset between us.