He should get a DUI regardless. Even if the cops pretend to believe his lie about not being drunk, being under the influence of drugs is also grounds for a DUI, and he has admitted that.
I agree, but unfortunately there's no physical proof of intoxication.
1) Was driving with his headlights off at 2:45 a.m.
2) Swerved into oncoming traffic, almost sideswiped a police cruiser, and crashed into a security barrier.
3) Was observed staggering after he exited his vehicle.
4) Smelled of alcohol.
5) Lied about being "late for a vote".
6) Lied about not having had alcohol,
7) Was seen drinking at one or two bars earlier in the evening.
I'd say that the above facts sufficiently form the basis of a case. I'm sure that the two sergeants who drove him home could provide testimony that would ice the case. No need for any test results.