Firstly, don't panic! I know as I did.
My DD aged 16 was admitted to hospital on Tuesday morning following blood tests. I'd been suspicious for a while that she might have diabetes. I was diagnosed type 2 (tablets and diet controlled) in February. My mum had type 1 and died as a result (unable to control it) and most of my female relatives had it. Its normally late onset, ie 40's but I had serious concerns during the recent half-term when I saw just how much my DD was drinking.
So, two weeks ago I took her to the GP who organised a blood test. She had that last Thursday and on Monday am I had a phone call from the GP who wanted her to go in to have her urine tested. As DD is at a college that is not local I said I'd take a specimen in to the surgery but the dr was very insistent that DD go in with me and the specimen and wait until it had been 'dip tested'. Literally within minutes of leaving the pot in reception we'd been called through to see the nurse who tested her blood (fingerprick then glucose test) and then the dr was in and we were sent straight to hospital.
She clearly had type 1 diabetes and it was confirmed pretty soon after arrival. Her blood sugars were 34 which is dangerously high. (in the morning I believe they should be between around 4 - 7 before food) The dr in the ED could not believe she hadn't collapsed. She was put on a sliding scale (insulin in a drip form) and kept in for 2 days. She saw our diabetic dr yesterday and is now testing and injecting 4 times a day.
That is very very rare. I don't want you to worry and don't want the comments above to scare you.
When I was diagnosed, I had the first blood test then a second blood test which picked up that my sugar was high and I was type 2 diabetic and only needed a careful change of diet and tablets. I asked the dr on Monday why DD was not having the second blood test and she said there was no need as it was so clear from the first that her blood sugars were sufficiently high for them to be sure it was diabetes. I believe (although could be wrong) that the second test is done on borderline cases, which is probably the case in your DS's case. He may just have slightly raised sugar which may only mean a change of diet or possibly medication (I am on metformin tablets). Shortly after I was diagnosed I went on a course with about 10 other newly diagnosed type 2 and the majority were not on any medication, just diet controlled.
So, please do not fret. I'm pretty sure that if it was serious (ie suspected type 1) they would have done an immediate test on his urine (for keytones which is a bad sign). The fact that they haven't is a very good sign and I'm sure it will just be slighly high sugar that you need to keep an eye on.
The tiredness is a very common sign of diabetes (type 1 and 2) as both myself and DD had it. Please try not to worry and keep in mind that if they were really worried, then you would not be waiting for further tests, they would have done them immediately x