Does he have any interest at all in science? I have tutored some pupils who were set to do Foundation all along, and then part way through year 10 (or later) started looking more closely at sixth form options, to find that the chosen sixth form requires a 6 in maths for all the science subjects. I don't know if that is common, though. It's been disappointing for those children, as they sometimes have more motivation at that point, having some idea of what they might want to work towards, and also often bright enough that they potentially could get a 6, but there is just too much work to catch up on to make it feasible for most of them. With the questions including more 'problem solving' ones that integrate different topics, it can be hard to try to teach them just enough that they could get more marks and achieve a 6, because separating out the material into different grades doesn't entirely work (obviously some topics are clearly harder or easier, but not everything).
So depending on how setting works at your school, and whether it's likely that he might develop a later interest in some science subject, it might be better to stay in a set that has an option of doing higher, even if he doesn't ultimately do it, at least for a while.
A good chunk of the foundation paper actually has stuff that has been taught since about Year 6/7 - all the basic whole number, fraction and decimal calculations (+ - x /), finding fractions or percents of things, place value, areas perimeters, straightforward word problems, simple ratio, basic co-ordinates, properties of shapes, etc. If a child is secure on all those topics, then they might enjoy the challenge of learning the harder foundation content and onto the higher. But if any of the basic skills like that are missing, especially if they've been reviewed each year, then Foundation is likely to be more useful - those are many of the skills that are actually helpful in life, and it's better for a child to really understand what fractions are, or how to find a percent, etc, than to learn trigonometry or factorising or whatever, even when those topics aren't necessarily hard to learn how to do (they can be a bit meaningless for some Foundation students, even if they know what the steps are).
You could certainly look at a Foundation tier paper with him now, and make sure that he can get all the basic questions and the problem solving using those basics, and that might help you decide.
Bring back the intermediate tier! And intermediate sets.