Again don't panic, but be flexible.
DD is dyslexic with slow processing speeds, and so was never going to do well on what is a speed test. Similar problems affect those for whom English is a second, albeit fluent language, including Welsh speakers.
First of all put together a spreadsheet of the requirements of different Universities, and see where he might qualify. Then see it as a series of hurdles. He may not be able to hit a high bar on one (I think some mixed metaphors creeping in) so he needs to do better on others (work experience, academics, personal statement, interview.)
Medical and dental schools cannot all recruit the same applicants, and UCAT is not a particularly accurate predictor of future performance at medical school. Some will want to venture beyond the top UCAT performers to identify good potential students.
Further tips.
Keep him away from TSR and MN. There will be lots of people boasting about 4 offers when your DC may be waiting on his first/only interview. It is upsetting, and people are really tactless, but if he gets his place, he has done just as well. You can only take up one place, and once in, no one will ever ask his UCAT score again.
Treat it as a two year process. And expect to wait. DD had three interviews in March, when all her classmates were sitting on offers and planning their accommodation. Luckily she got two, but was so focussed on her gap year plans she deferred anyway. The two friends in a similar position (they had resulted to sitting together in the sixth form common room to avoid the rest) both failed to get a place, but were the first to congratulate her. One got a place the following year (and is now performing at the top of her year at one of the more academic medical schools) whilst the other realised during his gap year that is was probably his Doctor parents who were keen on medicine rather than him. (He got three promotions in his gap year job and was offered a further large promotion if he was willing to skip University!) The first year is good experience for a reapplication, so he should go into the test not worrying about whether he will be good enough, but seeing how good he can be under test conditions. (Oh and take some ear plugs with him to help concentration.)
Practice, practice, practice. He won't have much time left this year, but start early next. Think about getting him some tuition, either a course or 1 to 1. Good technique helps. We knew a boy who started practicing in January, about an hour a day, and got somewhere in the top 2%. (He also had a mum who phoned out of the blue to tell me his score despite knowing that DD was likely to have done far less well, but these types exist IRL as well as online! She was slightly quieter when her DS failed one of his first year exams and had to resit, but hey, medicine/dentistry are long courses and UCAT is not a great predictor.)
Good luck.