Hi OP
If your son is applying for Harvard etc in the US, realistically, in the U.K, it's only worth coming over for unis with a comparative global reputation - ie
Oxford or Cambridge
Imperial
UCL
All these are top 10 global institutions, alongside MIT, Harvard, ETH Zurich, as I'm sure you know.
If he was interested in Economics, there is also LSE which, although not ranking particularly highly in QS (for some reason), still is a recognised name.
If he gets into Harvard or similar, let's be honest, he's not going to trade that to come over here for the likes of Warwick, Bristol, Durham etc.
You mention St Andrews - seems to be well-known by Americans. It is small and self-contained though and obviously quite far north / remote. Could he cope with that for 4 years?
The most important thing at this stage, obviously, is for him to decide on a degree subject.
If it's sciences or Engineering, look at what's in offer at Cambridge, Oxford, Imperial, UCL. There are only so many options.
If Economics, look at those 4 plus LSE.
Then, in terms of a consultant, you could contact Crimson Education, Oxbridge Admissions or U2 (they deal with Oxbridge, I think). All if these can give PS advice, mock interviews, subject mentoring etc.
I'm sure you know that the way the PS is written over here is VERY different to the US. They don't give a hoot about being president of the football team, or whatever - it has to be ALL subject specific for Oxford, Cambridge, Imperial and LSE. UCL may be a bit more interested in extra-curricular, but only marginally.
DS has just graduated from Cambs. There were international students there who used consultants, so your question is not unusual. I have no personal experience of using them - however, when DS started out, I did ring Oxbridge Admissions (the consultancy company, not admissions at the actual unis), and found them very helpful. I had a couple of hours chat for free, which was helpful as I was totally clueless and it's a steep learning curve. You only pay if you sign up to one of their programmes.
Good luck!