Well he probably doesn't know how much groceries cost really (not making excuses for him obviously) - it's not like he's frugal when it comes to spending on himself so just chucks things in the basket without worrying too much about it.
Remember his goal is to force you back out to work so he can stop supporting you entirely, as his father did to his mother. So trying to shame you into feeling that you're stealing from him is as effective as actually believing that you are. In fact I guess it's the same thing to him - by taking his money at all you are stealing from him.
Isn't his own phone not working due to whatever the problem is?