mc7

Is the only problem that it's still healing? (That it's taking so long?) A couple more weeks of healing is probably pretty normal considering it's 6-8 weeks to heal from circumcision. If there's any itching or inflammation like infection, that's different.

He might avoid all dairy (except maybe yogurt) until it's healed to minimize infection risk. And obviously sugar/sweets. It may help his kidney function (esp helpful for diabetes) as well as minimize infection risk to consume some form of cranberry probably at least once a week until it's better. There are sugar free tablets. Or even better, you could sweeten fresh cranberries (prepared or juiced) with a sweetener like stevia. There seem to be some studies showing some benefit of cranberry for diabetes generally.

Part of the reason I'm mentioning infection minimizing things is because yeast infection might have been the reason the skin tightened requiring the slit. And UTIs--yeast and bacterial--are common problems with diabetes.