Firstly, I am sorry to hear your son is struggling. Please be assured, from someone who has taught A-level for a long time, many students find it challenging but go on to be successful.
You have probably done this but a first step would be to speak to his teachers at school. He needs a very specific list of things to work on, perhaps week by week to make it manageable. I did this for a student once - almost a tick list of things he could work through and show me.
Are there support sessions available that he can go to? If so, he should go to these.
What degree does he want to do? Many offer a foundation year for those who don't have the required qualifications for whatever reason. This could be worth a look - if he already has offers, contact admissions. He still has time if he has not submitted his ucas form so look into foundation years for him.
Also, consider carefully where he is applying- would a smaller uni be more appropriate rather than a huge one where he will get lost in a crowd. Support him to investigate what support is available for those with additional needs as this can vary massively.
For what it's worth, I taught a lad a couple of years back for A-level physics. He struggled massively with all of his A-levels but was adamant he wanted to read Physics at uni. He got a D, went to do Physics with a foundation year and is now absolutely thriving! This was a boy who couldn't even organise his folder! So there is hope - some find it clicks later!
Best of luck!