Starting in semester 2 ... will I fall back? (2 Viewers)

[Absolutezero]

If he does 4 in S1 though then he 'catches up' 1 subjects on everyone else who started half a year earlier than him.

I guess so, but it's still unnecessary.

If OP thinks they can handle for, then they may as well. However, it's not necessary to do, and you'll be fine if you don't

 

[OVYO]

This is no longer school, you will mix with people of various ages and you will often find people from other streams doing the same subject as you and often from other years. I've done subjects where some people were in their 5th year doing 2nd year subjects and still not completed a 1st year subject, people are all over the shop.