Dog obsessed with sons smell

I have a lab who is 18m and 2 sons 8 and 5. The dog is primarily my husbands and he does all the feeding and walking. He is generally a lovely dog at home with family and those he knows however he is anxious in nature and takes a while with new people. The vet said that because he is so anxious they wouldnt recommend getting him neutered at this time.

He has always been interested in the smell of my youngest son, literally follows him about smelling his bottom. We have corrected this behaviour and doubled down on hygiene with my son to stop it being so appealing. However he now seeks out my sons underwear and trousers and licks and chews the crotch. He never does it when he is supervised but when he is alone (only for 2 x 45 minute slots most days) he will go throughout the washing basket and take out my youngests underwear. We do crate but starting to transition out of this at night.

Anyone have any suggestions? I can remove the items from his reach but there will be some that slip through the net in a busy house but im more worried about the behaviour and that im missing some warning sign that might escalate. He is generally a dog that prefers adults and will sit with us over the kids but is very happy in the kids presence and is gentle to them.