Hi Sandra and welcome to the forum.
I have a 22 yr old daughter who is on DLA high rate for both, also a lifetime award. We were advised to apply for ESA when she was 18 which we duly did, she had to attend the medical. I'm afraid DLA doesn't come into it, they don't take that into account.
ATOS made us wait at the centre (a disused jobcentre) for a long time as they "hadn't realised" that a doctor would be needed, rather than a nurse, to do the examination.
Doctor seemed friendly, I had bought all medications including insulin along to show them. She asked numerous questions, tapped on the computer a lot and said that she didn't need my daughter to do the physical stuff, raising arms, getting up and down etc, it wasn't necessary. It could have had something to do with the fact that daughter had got quite het up at the wait and looked like she might explode..she was very upset.
Upshot was doc looked me straight in the eye, patted me on the arm and said "don't worry my dear, there will be nothing for you to worry about"
2 weeks later I got the letter that informed me that although daughter had been awarded ESA, they considered her able to do "work related activities" attending job centre interviews etc. I appealed this decision and got supporting letters from her tutor at college, her GP, her consultant at the hospital, her psychology team etc. I lost count of the phone calls I had to make.
I found out that I was entitled to ask for a copy of the doctors report from the medical, when I received that I discovered that "nice" doctor had awarded my daughter only 15 points, for continence issues. She stated that she had no physical mobility problems at all because she observed her "walking from one room to another on a carpeted surface". She lied that tests had been carried out, lied that my daughter was wearing glasses, that she had checked her heart, blood pressure and hearing and so on.
I sought help from the CAB who were useless I'm afraid and I took on the fight myself, after 6 months of hell, we won and daughter was placed in the support group.
I take no pleasure in recalling what happened and I promise I am not one of those who revels in imparting gloomy news but in this case, I really do suggest you are prepared for any problems. Depending on if you believe your son can work (and I'm guessing that's not the case) I would actually work on the assumption that he is going to be placed in the WRAG group (work activities) and get supportive letters from who you can ready, just in case. Be truthful but don't say that he just can't do things, they will want to know exactly WHY he can't do them. Get some support with an appeal, perhaps ring up the CUK adviceline (number on the main homepage) on what to do if that happens.
Above all, don't expect them to think as we do, they don't see common sense, they are ticking boxes and trying to meet government quotas, no matter how much they say they aren't. But don't give up, whatever you do..appeals are often successful but you need to stay determined because they will do their utmost to put you off.
Good luck