If you are driving long distances in the summer you might want some sort of cold box to keep the insulin cool.
If your DC is going to be with you at all times then maybe no need for this, but DH has got a wristband stating that he has T1 diabetes just in case he starts a hypo or is involved in some sort of other medical emergency and I'm not with him (eg road accident).
Also watch out for the change in routines, if your DS is used to eating at fairly regular times and the routines are relaxed on holiday it can mess up blood sugar levels, as can extreme heat or more exercise (walking etc) than usual.
We were in this situation earlier this year the first time we flew after DD's diagnosis. We flew and I was worried about needles at the airport so I checked with the security there in advance and they said no problem and on the day of travel they didn't even want to check it. I had a copy of her prescription as recommended and they didn't even ask for it, hundreds of thousands of passengers with diabetes travel every year and they are used to it at security, whether it be at airports or other ports.
Make sure you have enough for the whole holiday and spare. If flying, pack in hand luggage, not hold baggage in case it goes missing.
Agree with suggestion about snacks, ALWAYS make sure you have quick release sugar, ie small cartons of fruit juice, glucose tablets etc.
Take a couple of copies of the prescription and the full name, address and phone number of Doctor. Keep one with his passport and documents, keep one in the car and keep one with his medicines. I've travelled with my Mum who has been type 1 for many years. I don't recall any problems with border crossings with needles. Even with airport security she has just shown her bag with insulin and needles and been waved through. When you get to where you are staying, locate the nearest A and E hospital and look up the word diabetes in all languages you may need. Make sure you have a stash of snacks in case needed or have a look in the local supermarkets for what to buy. I was in Berlin with Mum one year. I woke up in the middle of the night and realised she was ill, thankfully there was a 24 hour petrol station next door to the hotel, so I ran down there and bought an assortment of peanuts, pastries, carton of juice and breadsticks. When I got back she was waking up and checking her levels.
DSS was diagnosed with Type 1 diabetes last year. This year will be the first year that we have travelled abroad since diagnosis. Can anyone tell me what we need to take in case of checks at country borders? Do we need a letter from GP confirming diagnosis and medication? Will there be any issues transporting needles and insulin? I have looked on the Diabetes UK website but found it quite vague. We are driving to Germany via France and Belgium possibly with a day in Luxembourg so no flying involved. Any other tips on what we should take / be aware of gratefully received. We have appropriate travel insurance and a European Health Card. Many thanks.