I would interpret your DH's behaviour as passive-aggressive sabotage of your plans for this trip. He has deliberately rendered himself unable to drive, knowing that you can't either, legally.
I doubt you'll be able to add yourself to his insurance unless you can fake a baritone and impersonate him on the phone; it needs to be the policy holder.
There's no point choosing this battle in the morning with him hungover and your mum coming as a passenger, and he could well be counting on your mum as protection from your potential wrath.
How keen was he on this weekend away? Has he openly expressed either enthusiasm or reluctance and would he feel comfortable to if it were the latter?
I'd suggest plan A is get everything ready for you and your DC, have the insurance papers handy, have him ring his insurers and add you as named driver (assuming you haven't been left off because you are a bad risk with lots of points on your licence). You then drive. 3 or 4 hours is too long but take a break halfway and go steady. You're way safer than he would be behind the wheel!
Plan B If he starts to say he's fine to drive and won't call, go out of earshot and impersonate him! My insurer routinely lets me add another driver without charge but I have half a century of no claims to help. I'd get a week or more, not a day; cover the whole holiday in case he decides to spend it self-medicating with alcohol; I wouldn't tell him how long you're covered straight away in case that looks like an invitation! Proceed on holiday.
Plan C, once insured is go without him if he sabotages your start time by acting comatose. Roll him into recovery position in case he voms and leave him to it. Only if you're happy he's just hungover and deliberately being awkward, naturally. 7 pints shouldn't be life threatening, though anything above 5 probably stopped him counting accurately anyway.
Plan D would involve an ambulance and cancellation of holiday but he's probably not going to need that - at least not because of what he drank last night. D had the alternative ending of a new patio, I guess.
Later - much later after you get home - you need to have a conversation with him about sabotaging family plans and how he could have handled things differently and better if he didn't want to go!
Is this the first time he's pulled a stunt like this or does he routinely exert control through sabotaging previously agreed plans? Has he manipulated the insurance situation so he can control where you can and can't go as a family?
He's been selfish and thoughtless at best this time, at worst dishonest and manipulative.