Hmm I honestly don't know how they'll consider GCSEs taken this year, given that the exams never actually happened. Schools and colleges are saying very clearly that transcripts for this period will have notes on them pointing out that there was a pandemic underway though.
Having said that, in normal circumstances they would be looking for excellent GCSEs, and predicted A/A*s at A level. It sounds like he'll ace the SAT which will help, but he'll be up against lots of other kids who have also aced it. He should also plan to take some SAT subject tests - DD1 took Physics, Math 2 and English.
You can contact the admissions office of anywhere he's interested in, and ask them. They're usually well-staffed and fairly responsive, although as most of them are working from home it's worth emailing rather than calling.
His out-of-school life is also really important. This page is good to see the kind of thing the Ivies are looking for
www.harvard-ukadmissions.co.uk/applying-from-the-uk/
"The US system in general, and Harvard in particular, look for a wider range of interests and pursuits in an applicant than the British system, whether activities that you have been involved in to a high level for some time, or new interests that have recently caught your fancy and that you have put time into. In some cases it may be a sport that you have taken to advanced level, or developing your talents in the arts, music or theatre. You may have been learning a language just for fun, or working in a community service activity.
You may have won prizes for Science competitions or tutored primary school kids in Maths. You may even have started up a small business and shown genuine entrepreneurial spirit. All these things are part of ‘who you are’ as a person, and US unis look at the whole person when they make their decisions, not just your academic grades. So when in Year 11 and Lower VI, take time to focus on what you have accomplished so far, and think about what aspects of yourself you’d like to highlight. It may help you decide what extracurricular activities you decide to keep up and develop further, whether during the school year or in the holidays."