IMO B grade is fine for all of those subjects. Presuming of course he didn't/doesn't work very hard to achieve it, and is willing to put in some more work at A-level.
My advice would be for him to take all 4, and see how he gets on.
I started of doing 4.5 A-levels (I don't really count critical thinking as we were forced to start it, and only a handful continued it when we gained the option to quit), and found that I still had a lot of free time, so picked up Chemistry as a 5th subject.
The second year is tougher, so I dropped down to 3 A-levels, which is all you need for university admittance.
My advice would be for him to take all 4, and see how he gets on.
I started of doing 4.5 A-levels (I don't really count critical thinking as we were forced to start it, and only a handful continued it when we gained the option to quit), and found that I still had a lot of free time, so picked up Chemistry as a 5th subject.
The second year is tougher, so I dropped down to 3 A-levels, which is all you need for university admittance.