Hello, and welcome!
My name is XXXXX XXXXX it will be my pleasure to assist you and Booker today.
I'm sorry to hear he has been dignosed with diabetes, but it sounds like the insulin is working, if he went down to 329 after his blood glucose was so high. He will need to be stabilized on the correct dosage of insulin, with the vet's help and sometimes this can take few weeks or a few months.
As you may know, untreated diabetes causes excess drinking and urination. Actually he's not peeing more because he's drinking more; just the opposite--because he's peeing more, he needs to drink more. However, now that his blood glucose has come down significantly, he should start improving with the diabetic symptoms.
First, you will need to clean the spot where he urinates with an enzyme based cleaner, which will completely remove the stain and odor. Some well recommended ones available in pet supply stores and sometimes, Target, KMart, Walmart are Nature's Miracle, Zero Odor and Odor Mute. I would also suggest spraying some Feliway, a cat appeasing pheromone, in this spot after it's completely dry from the cleaning. In addition, don't allow him access to the area until the Feliway is completely dry--about 20 minutes.This will make him think he 'marked' there already, so he won't need to do it again. Here is more information on Feliway spray and plug in diffusers, which you might use in that room, as well: Feliway
Add another box or two in different locations in the house and show him where they are.
Also, add some 'Cat Attract' kitty litter, an herbal additive which encourages use of the litterbox. You can find it in any pet supply store. If his current box is covered, uncover it. If you've changed litters recently, go back to the old litter. Make sure the boxes are not near any high traffic areas of the house or near any noisy appliances.
Cats are creatures of habit, so you will have to help him break this bad habit with these suggestions and hopefully, he will start using the litterbox for urination, again. As a last resort, you can discuss with your vet, a prescription medication to help with inappropriate elimination.
I hope all will be well with your Booker!