Excessive weeing

[Dogmum94]

Hoping someone else may have been through similar and could suggest things we haven't thought of!
Ddog is a 15 month old Lab x Springer, was on Autarky working dog since we got him from the breeders. Around 10 weeks ago we noticed he had started losing weight, his poos had become really sloppy and unformed and he was weeing lots throughout the night even after not drinking anything before going to bed.
Vets thought it could have been a few things - an obstruction (nothing found), an infection (nothing showing on bloods) or developing an allergy to the food he was on. Changed him over to a food for sensitive stomachs and the change was almost instant. He's got back to a healthy weight and his poos are all solid for the first time in ages. Vets were happy and said it was quite an unusual reaction for allergies but believed it must have been as the food stopped the issues.
But now for the last few days he's gone back to weeing excessively during the night. During the day it seems average as he has access to water all the time and he'll drink whenever he needs to after his meals. He can control it as he will wake us up to be let out rather than just going in the house but last night he went out 7 times and did a proper wee each time. No other obvious signs of a UTI.
We just have no idea what could be causing it or what to even begin looking at?