Welcome, Terrence!
Lantus is a great insulin, so you have a great start! Would it be possible to just get rid of the dry food and feed all of your cats the pates? Then you don't have to worry about him getting into it at all, and non-diabetic cats typically do better on canned food, too! It helps prevent diabetes, urinary tract disease, and kidney disease to make the switch. Some people with multiple cats free feed the canned like dry (you just add water to keep it moist longer), and/or freeze canned portions for autofeeders. I have two cats, one diabetic and one not, and they both eat the same canned foods, and I use two autofeeders to release the food at meal times when I'm not home to feed them.
If you have a hardcore dry eater that won't eat canned, this website has some great advice:
http://catinfo.org/#Transitioning_Dry_Food_Addicts_to_Canned_Food_. There are also some lower carb dry foods for the die hard dry addicts while they transition, like EVO or Young Again Zero Carb, that at least would minimize the impact on Terrance's blood sugar if he manages to break into it. I only recommend these to people if their cats absolutely refuse canned, because lower carb dry foods still cause other health problems.
Are you home testing yet? Home testing is very important in getting your cat regulated! It looks hard, but nearly all cats will accept it with a little time and bribery (with diabetic safe treats, of course)!

Starting doses of 5u and 3u of insulin are a bit high for a cat (the most Bandit has ever needed is 1.5u, I believe). How did you get up to that high of a dose? Dosing with Lantus is typically based of the daily nadir (the lowest number of the cycle). If you're not getting that test, too much insulin looks exactly the same as too little insulin when you do weekly or monthly curves.
If you have any questions at all (and there's a lot of info out there so don't be afraid to ask!), please post over in the Health forum. With the right food, insulin, and dose adjustments via daily home testing, 80+% of newly diagnosed cats go into remission, and no longer need insulin. Wishing the best for you and Terrence!