From my experience on uni (I've been to few) there is always one opportunity to resit, normally with the grades capped at a minimum pass. E.g. you might score 80% on the resit but you will only obtain 40% or pass not merit or distinction.
If he fails the resit then there should be an opportunity to resit the year / modules. If it is only two he has missed he might be allowed to just study those two modules. This would make him a part time student with implications for finance.
Depending on which modules he has passed he might be able to take some year 3 modules.
I did my degree part time and in a joint honours so at some points I was taking year 3 modules in subject A but year 2 in subject B.
The options of what to study will depend on the prerequisites he has passed. So say there are 3 maths modules and you have to pass maths 1 to take maths 2 and you have to pass maths 2 to study maths 3.
So if he has passed, in my example Maths 2 then he should be able to take maths 3 but if one of his fails is maths 2 he won't be able to progress.
As a PP said it does depend on the rules in the individual university so please don't get excited about what I say next. Sometimes it is possible to to take modules from different discipline.
*I'm using 'module here' but each uni uses different terms, some call them units.
BTW I have the upmost sympathy for your son with the group work, it can be a nightmare, I did one in my second year, out of 4 of us, one left the course, I became homeless and 1 had to bring her children.
In the end we got a really high mark but the one with child care issues complained.
In my third year when I had group work I started out by saying, "If you let me lead this, we will get an A", I did and we did.
I think with group work at UG level you want to work like democracy, and that doesn't work.