I'm sure if the boys carry on helping their parents with their work, the parents will want them to make themselves useful and won't leave them without the skills to do the job properly. That wouldn't be in anyone's interests. So I imagine you are somehow missing part of the picture here.
Maybe they find maths very difficult: my learning disabled 20yo certainly needs me at her elbow whenever she measures. She's slowly heading in the right direction, and luckily there is no deadline! Or maybe they were taken out of school in recent years after getting the message there that maths is hateful and difficult - that loss of confidence can take a long time to overcome. Maybe these lads simply aren't interested yet, but they will see the point as soon as they want to work for themselves. Learning follows on very quickly once the motivation is there.
(It is of course illegal for young teens to help out with their parents' work if it takes place during the school day or for too many hours per week, so if they are doing that then their parents are on the wrong side of the law.)
You asked about GCSEs. There is no requirement for home educated teens to take GCSEs. Most who are capable of taking them do so, either because their parents want them to or because it's necessary to what they want next, such as college. They don't tend to do a large number of them all in one sitting as schoolchildren are made to do. Instead they may spread them out over several years so they can focus on a few subjects at a time and reduce stress. A very academic child may dip a toe in the water with a single subject at 12 or 13 and then do a few every year thereafter. A child who finds exams harder might do two or three at 16 and a few more at 17. Five or six well chosen subjects is usually enough to get them into college or their preferred career.
For those who have no GCSEs, colleges usually have something available, because of course many schoolchildren also don't pass their GCSEs but everyone is meant to remain in education or training until 18, so in theory there should be something available. There are generally some Level 1 courses available to teens with no qualifications. Maths and English GCSE can always be done at college post-16; in fact current government policy is that adults of any age can take those subjects at college for free if they don't already have them. Some colleges offer a fixed set of six or so "catch-up" GCSEs to teens who don't have them, though usually there is no choice of subjects. But it's something.
Some kids go straight into work without qualifications. This was the original plan for one of my kids, who was capable of sitting GCSEs but didn't want to. I saw no need to force them, since I knew they could do them later if they changed their mind. For home educated young people, there's no particular reason to do them at 16 rather than any other age: they have to pay regardless. Instead, my child did a lot of part time paid and voluntary work in their early teens in order to gain experience and build a good CV, alongside learning about everything that interested them, with no immediate intention to sit exams.
Unfortunately in their late teens my child became too ill to do any regularly scheduled work at all (chronic fatigue). They decided their only option was part time academic study, which they could manage whenever they felt up to it. They did English GCSE at 19 and maths at 20 in parallel with self-directed art study and some paid and voluntary freelance art work, then went to uni part time, emerging with a good degree at 25. Getting in on portfolio and relevant experience alone was only possible due to it being an arts course. We do know people who have done Access courses to get into Uni without previous exam passes, and others who have gone straight to work - some of whom later returned to education as mature students in order to change career. They tend to be quite focused because they've been allowed to make their own life choices from an early age.
So yes, it's not ideal to bypass GCSEs altogether unless you know your career won't need them, but other paths are available.
I don't regret not making my child do GCSEs at 16. I do regret not leaning on them to pass their driving test at the earliest possible age. This has proved to be a bigger practical challenge and continues to restrict their job prospects. It intrigues me that so many people believe all kids should be made to do GCSEs young "just in case", but don't have the same attitude to driving.