It makes sense to me!
The maths modules are as follows:
C1, C2, C3, C4 (core or pure maths)
M1, M2, (M3 & M4) (mechanics)
S1, S2, (S3 & S4) (statistics)
D1, D2 & (D3)
FP1, FP2, FP3 (further pure)
All the boards have a similar structure - I've put the less commonly taken ones in brackets - they are not necessarily available on all boards. The lower numbered modules in any area have to be taken before the higher ones and they get progressively harder. I think for FP2 & FP3 some at least of the core modules are also prerequisites, but for FP1 on most boards it can be taken
For AS maths you need C1, C2 and either M1, S1 or D1.
For A2 maths you add C3, C4 and another M,S or D module.
For AS f. maths you need FP1 and two more modules
For A2 f. maths you need 3 more modules numbered 2 or higher including either or both FP2 & 3 (slightly board specific, sometimes FP2 isn't a prerequisite of 3)
Often for A level maths people choose C1-4, M1 and S1 as the 1s are easier. And a typical f. maths A level would be FP1, FP2, M2, S2, D1, D2 (maybe replacing D2 with M3,S3 or FP3).
He may well find FP1 easier than mechanics if he doesn't like mechanics, but he will find M2 harder than M1. There is a degree of 'getting it' though - so if he persists he could find it all falls into place & seems straighforward. Maths is like that.
I imagine he could either drop f. maths now and just do a single maths A level, or plan to just take f. maths to AS level by doing all the modules this year as planned and then just 3 more next year.