now at uni one week down, when can I relax?

[Sounbelievablydull]

Context, my DS did very badly at as and we honestly thought he'd be going no where this year. However he managed a huge improvement and despite not quite getting the offer grades got into his first choice ,russell group uni.
Now I feel anxious that he will lapse back into his as ways as there is a definite streak of complacency there and no one there to nag.
I'm not too sure what I can do about this but just wondered whether anyone had any advice?

Difficult, as they're far away from you, but I've been reminding DD that she's paying £9,000 for the pleasure of being at uni, to work hard and do her best and try to keep ahead of the game with good preparation for lectures and seminars. Freshers' week works well if only to get all the mad partyting out of the way, such that they're desperate to settle down for an early night with a good (text)book.

I'm not sure what you can do - he's an adult, living his own life.
Just emphasise the fact that University is not like school. At school you can wing it and get good grades by IQ. At University you only get good grades by hard work.
Having said that, at most places the first year is not that important, it's sufficient just to pass.
Ask interested questions about how it's going on but, really, it's down to him now. (Generalisation alert but) a lot of boys mature late but do get there eventually!

Hopefully his first year will be just a pass required (most are, although not all) so you can let him learn the ropes about hard work. He might also get more regular feedback this year in that essays etc are often marked as you go through which gives some sort of guidance as to how he is doing and might act as a spur.

This is where he begins to learn that work gives results and that he has to be independent to survive in real life.

I share your fears OP as my DS did exactly the same as yours - had a wake up call after AS results were'nt as expected, pulled his socks up a bit and got into his 1st choice RG Uni even though he dropped a grade in his least important subject.
It is out of our hands now though. I will no longer have a clue how DS is doing - it is all down to him. He will be the one with a massive debt at the end of the course - or sooner if he doesn't last the course.

I am worried about this myself. Ds got into his first choice, but only just. Didn't quite get the required grades but got in. I wrote on the 30 day only bit about what he did!
So without me to prod him, I'm concerned he will mess about gaming all night etc and will be kicked out after the first year. I won't have a clue what's going on as he is "supposed to be an adult" now so the days of reports to parents are gone.
I only hope that because its a subject he enjoys he will take it seriously.

If you are still worrying now, i think the honest answer is when you die

Step back! He has to do the work & find his own discipline now. To support him, support his tutors. They'll be on his case if he slacks off too much.