If he's willing to work, I'd suggest you go backwards and get him to do past papers/revision guides/workbooks at a lower level to improve his confidence and fill any gaps in his knowledge/ability.
Maths is a subject that progresses at quite a pace, so if he's struggling with the basics, he's going to struggle as the course progresses, not just getting further behind, but also knocking his confidence.
It would be good if you/he could drag him up in the next few months, to improve his basic maths skills ready for the new school year in September so that he can hit the ground running and hopefully keep up during lessons, so that he doesn't fall further behind.
There are lots of free resources online. Have a look at CorbettMaths which have plenty of worksheets but also online tutorials showing how to do things. There are usually also loads of used books on ebay and even plenty of relatively cheap new workbooks on Amazon.
If you start now, you're willing to sit with him and help him/make sure he does it, and he's willing to do the worksheets etc., then, yes, every chance he can improve by several grades.