It's all well and good for unis to say the first year doesn't count because it doesn't count to the over all classification but in some courses the marks in the first year decide if they get to progress. It's not always explicitly stated, it's assumed students read the course handbook and know this
We don't say "first year doesn't count". We say that the marks you receive in first will not count to the overall classification of your degree. But you have to do well enough to progress from first to second year. Please don't try to shift responsibility away from students to take responsibility for completing the required work for their degree!
This is why transfer between different degree programmes or subjects between 1st and 2nd year usually very difficult or impossible.
And we do assume that students read the course handbook, so they are fully informed about the conditions of the course for which (as many of them sometimes quite rudely remind us) they are paying. It is the student's responsibility to read the handbook, and know what they're supposed to be doing. It's not school. In 3 or 4 years, undergraduates are expected to be able to work as professionals.
Sounds to me, OP ., as if your DS has had a bit of an adjustment problem between school and university. It happens. Particularly if school & A levels were relatively easy. And Engineering is a notoriously heavy course. If he gets through, he's learned a valuable lesson about consistent and diligent work.
If he ploughs one exam, but overall passes the year, with passes in the right modules to maintain progression and satisfy external certification bodies, then he's likely not to have to resit in the summer.
If, however, he hasn't done the work, and can't progress, he'll be offered re-sits in August, generally capped at 40% (which is the bottom pass mark for an Honours degree). Some degree programmes allow students to fail (or not complete) up to 20 credit points in a year, as long as their total credit point load over the whole undergraduate degree does not go below a specified threshold. The standard UK credit point load for an undergraduate year of study for an Honours degree is 120 credit points (an Honours degree totals 360 cp).
But it may be different for an Engineering degree, especially if it's an MEng (as opposed to BEng).
All of this will be set out in your DS's course handbook and/or the University examination & progression regulations.