No real magic
I took his end of year report and noted the long list of things he couldn't do, then I investigated various ways to reinforce the concepts.
Unfortunately he already knew he was crap at Maths due to something which happened in the classroom so I explained to him what we were going to do. I tried to empower him to believe he could change things, he wanted to be better at Maths and he wanted to enjoy it. His motivation was that he could get better, I could help but ultimately it was him that would change things - he liked that.
I changed the approach every day, using board games, cooking, physical games (score keeping), websites, practical counting involving smarties which would then be eaten. Or lego, pasta, beads etc
Some things he found very boring so I went away to think of other activities to reinforce. If he didn't get something immediately I explained it was because I hadn't found the right way to explain it yet, which was mostly the truth.
I kept a note of the things I did everyday, so as not to get into a rut and to make sure I was covering everything. We spent 10 mins a day.
By the end of the summer he was confident in Year 1 Maths, when he started in Sept he was more alert in class - he no longer felt out of his depth. He became very aware of his level of undersatnding and would come home and tell me when he didn't get something in class and we'd go over it at home.
Half way through the year we got Maths Whizz which assessed him way below his real level but I was more than happy with that as again he consolidated ideas and really, really enjoyed the games. I continued to work with him but was a little more hands off - that was his choice as he could now do it himself!
Going into Year 3 I will continue to take a keen interest in his Mathematical understanding, but I get the feeling that he will no longer just accept not understanding it at school - he knows that I'm here to explain a concept to him in a million different ways if needed.