My brother was the same a few years back, was spending all his wage the day he got paid, then helping himself to his mrs money and then using whatever means possible to hide it all until he had no way out. Unfortunately if it's your mum on her own trying to deal with it then it's difficult other than trying to limit his access to sites, but that's not a permanent solution and will probably just aggravate him more. We all sat down with my brother and made him aware of what he had to lose and where his life was heading, a bit of an ambush but desperate times and all that. Anyway I took him to GA every week for a while and that was the best form of treatment for it, he got to see people that had hit rock bottom and done things beyond comprehension to try and feed their habit, within a couple of sessions the shock had all but sorted him out but you need to persist with it so as not to slip back into bad habits. Me and his Mrs took control of all the money and his wages and basically dished out an allowance until we were convinced he was sorted and that worked perfectly, also made him realise how much he was wasting and he's now saved enough for a deposit on a place so a complete turnaround.
It's a tricky one if there's nobody able to do that sort of thing with him, we were fortunate in that my brother was receptive to the help rather than fight it, but just blocking sites isn't going to get you the results you want as there'll be other ways of gambling. Someone needs to have a serious word with him and grab him by the scruff of the neck if necessary until he understands the wider impact it's having on his family. It's a crap situation mate but being brutally honest with him is the best way forward.