Hmmm - he may feel slightly threatened by your diet?
If so - a nice romantic dinner is a clever response!
Don't turn it down - just try to Dukanise it as much as possible.
Chose steak or chicken or fish with salad or veg.
Try not to drink at all - but if you must, a glass of wine with dinner.
And if he wants to buy desert - whisper in his ear that you want to take him home early for an "special desert"....
Then, next day - pp day, lots of water, lots of walking (and I hope a huge smile on your face)
But next time - ask for flowers instead!