I can also see why this is a massive blow.
In the absence of any issues at home, it's likely that he's just had a really fun holiday, gained a closer relationship with his stepbrothers and during the holiday together they've had a bit of a bro-mance/lad time holiday.
I'd take a guess that this decision has a whole heap less to do with his Dad and more to do with the fun of hanging out with his stepbrothers i.e. similar aged lads who are into similar things etc.
Honestly OP, handled the right way and I reckon he'll be back by Christmas. All you need to do is remain cheery and light when you see him - big hugs, huge smiles, excitement to hear what he's been up to.
My guess is - something will happen, something in the relationship with his stepbrother - they'll get in a fight/argument/ something like that (which is inevitable when living with someone) and very suddenly living at his Dad's won't feel like 'his home' anymore.
So as long as you don't build in any obstacles for him to overcome (pride/ego/shame on his part) I think he'll be back.
Of course, all this is a guess - but if there's no wider issues for him at your house, then I doubt this has anything much to do with living with his Dad and everything to do with the lure of an older Stepbrother, and a new job.
That's just 18yrs olds - they go where the party is, and where the fun is and the minute the 'party' ends - they come home. I don't think he's gone to live with his Dad in the way you think. I would hazard a guess that his Dad hasn't been a huge factor at all in this decision.
I'm am outsider, don't know the whole story but that's just my initial gut reaction to it.
Like try and imagine you're 18yrs again - your parents are divorced. At home you have two teenage brothers who while you love them are still kinda annoying to you, at your other parents house you have 2 similar aged stepsisters - you've just had a brilliant holiday with them and one of them can get you a job working with her. You think she's super glamorous/fun/clever - whatever the lure would have been for you. In an evening you drink cocktails, and belly laugh at jokes. During that holiday she's been saying - why don't you come and live with us! It all sounds so exciting, doesn't it? But the minute that relationship turns even the slightest bit sour - what would you want to do? Go back home, where it's safe, to your bedroom, and your annoying brothers.....
That's what I think has happened for him in reverse and that's why I think he'll be back. So it's on you to keep your relationship with him a safe one, so that he can easily come home.
Maybe for whatever reason he doesn't, but in a few months you'll have adjusted too after this initial period of shock.
I guess overall stop looking at it as a huge rejection to you, your family and your parenting - like he's made a choice between his Mum and his Dad because in the words of an 18yr old 'it's just not that deep'.....
I honestly think he's made a snap decision off the back of a fun holiday. The fact he's bolted so quickly kinda tells me this is a FOMO situation.