This is not uncommon in Bichons.
The vet can either squeeze them to empty them, or you can do it yourself. I would suggest you take him to the vet to have the vet be sure there is no infection and to show you how to do this properly.
You will either need someone to hold the dog for you or possibly muzzle him to do it if you do it at home. I have done all our Bichons for my mum but I had to muzzle the first one as she was a biter. Essentially you get a large clean wad of cotton wool or kitchen roll or similar, place it over the glands and press firmly either side of the bum until the discharge comes out. You will probably have to squeeze more than once. Open a window nearby when you do this, the smell is appalling. Fairly obviously most dogs are not up for a firm bum squeeze which is another reason to have the vet do it. I have always done it as I am confident to do it and it saves a vet trip.
Once you have got it emptied, you need to look at putting more fibre into your little guy’s diet. It won’t completely stop the glands refilling, but you should find they need emptied less over. Mashed swede/turnip/carrot is a good choice. You can get it ready ,ashes and it will mix in easily with his dog food.
Good luck. The discharge really does smell abominable and full glands are very uncomfortable for him.