"He will easy be under limit by 10. Start feeding him coffee from 9 and let him drive."
No, he will not be.
If he has drunk 7 pints that is either 14 or 21 units depending on the strength of the beer. It will take between 14 and 21 hours to break this down and he only has 9 hours before they need to set out on their journey.
The NHS website states that "On average, it takes about 1 hour for your body to break down 1 unit of alcohol." However, this can vary, depending on your weight, sex, age, your metabolism, how much food you've eaten, type and strength of alcohol and whether you've taken medication and, if so, the type. It can also take longer if your liver isn't working normally.
There are roughly:
2.1 units in a standard glass (175ml) of average-strength wine (12%)
3 units in a large glass (250ml) of average-strength wine (12%)
2 units in a pint of low-strength lager, beer or cider (3.6%)
3 units in a pint of higher-strength lager, beer or cider (5.2%)
Coffee does not remove alcohol from your body. Only your liver can do that by metabolising it.
Caffeine can counteract the tiredness induced by alcohol but it can’t remove feelings of drunkenness or some of the cognitive deficits alcohol caused.
OP, your DH will definitely not be under the limit so it is good that you have decided to drive.