Hi, I’m a teacher (though just leaving the profession), but my kid is only 14 so I don’t have the complete experience yet.
I’d say that as with exercise, motivation fluctuates and the teenage brain hasn’t (usually) got the capacity to find that internal motivation so far out from the exams. Instead it’s routine and consistency that will make the difference.
I think you need to sit your teen down and explain that you want him(?) to be successful, and given the mock grades something has to change.
You’ve got to be prepared for this to be hard. He doesn’t yet have the frontal lobe development to manage this himself. On the one hand he’ll probably say yes, he wants your help and support, but then on the other every thing you do he’ll push back against.
If his phone is a big motivator (and the girlfriend), that’s your carrot to dangle. If you can’t physically take it from him, daily, then you’ll need to get onto the parental controls (there’s an app, Aura, that looks good) and lock it down overnight (as a minimum, sleep is so important here), as well as using it as the reward for doing daily, routine homework/revision/prep.
If I were you I’d start moving his wake up and bed time earlier by 5 mins each day until term starts.
You could also start with some light ‘revision’ - sit down and watch a YouTube revision video together (10 mins) each day at tea time or something. Start going over y10 work.
And then when term starts, you make sure homework gets done on the day it’s set, and then at least 30 mins of other revision each day.
Encourage this to happen early in the evening - just after school, and then he can get on to gaming/girlfriend after that.
Make a schedule for a subject per day: English on Monday, maths on Tuesday, etc. And find a section in a workbook or a YouTube video (but this must be engaged with - making notes/doing questions, etc).
Make this happen in a quiet but communal area in the house - in the kitchen while someone is cooking? In a dining room if you have one. And physically take the phone away until he shows you it’s done. To a decent standard.
Expect that he’ll do a poor token effort initially, or maybe once the novelty wears off, and you’ll have to be firm and he’ll be cross. Expect that it’ll go badly at times. But hold the line, he’ll thank you in the end.
Remember that it’s the consistency of the habit that is most important. Homework is first priority, revision is second. Something is better than nothing - it doesn’t have to be hours and hours.
I think this is what I will implement with my DS in September too. Good luck, OP!