I am a tablet controlled diabetic too. You would normally cut out sugar altogether and substitute it with artificial sweetener but I can't stand the taste of it so I still use sugar in my tea/coffee.
His sugar level will be higher then normal hence the need for the tablets to bring the level down.
He would benefit from eating brown wholemeal things instead of white (bread, pasta, rice etc...) as this slows down the process of sugars from the food being released. (check up on that)
Tiredness is actually one of the symptoms of diabetes so DO NOT give him extra sugar. Once his tablet dosage is sorted out, he'll feel like a new person with lots of energy!
A lot of it will be sensible, healthy eating, plenty of veg, fruit, baked instead of fried, lean food etc.... but you'll see for yourself.
A little of something that you fancy to eat won't do any harm - it's the control that's important.
You do not have to buy 'diabetic' labeled food - they're a rip off to be honest!
look at these sites
http://www.bbc.co.uk/health/conditions/diabete s/aboutdiabetes_symptoms.shtml
http://www.bbc.co.uk/food/recipes/mostof_diabe tes.shtml
http://www.google.co.uk/search?hl=en&q=food+a+ diabetic+should+eat&btnG=Search&meta=cr%3Dcoun tryUK%7CcountryGB
Hope I've been of some help - main thing do not worry