First off, I commend you for assisting him... I hope someone like you is around if I ever find myself in that man's situation.
If I were that man, given the situation (no glucose meter nor medical professional immediately available) I, as a diabetic, would want someone to try giving me a sugar boost. When blood sugar falls too low, it's amazing how quickly the symptoms clear by ingesting some carbs/sugar.
With a diabetic, especially elderly, the symptoms he was showing could easily have been low blood sugar or high blood sugar, or something entirely different (heart attack, stroke, or even intoxication, or embarassment at having fallen in public), so knowing he was diabetic and with medical help on the way I don't think any immediate harm would have resulted by giving him juice.
Thank you for your comment. If he would have been in a less responsive state, I would have insisted.
not really. it's important to be careful not to overtreat. ideally, a blood meter will always be handy, but if not, then administering 15g of carbohydrate (about a glass of juice) will do no immediate harm. in this case, it is unclear whether it would have helped, but it seems your instincts were correct.