I think the distinct 'averageness' of his average is going to probably wipe out 'good' traditional uni courses (I'm thinking things like, Engineering at Bath/Bristol/Exeter)? Is that a fair assumption?
I don't know about engineering admissions but you can see on individual course pages how much weight they give to GCSEs - Bristol is 20% GCSES, 80% A levels. Bath ask for quite a few at 7+. You can also contact individual uni admissions depts and ask whether his grades would be a deal breaker, they are usually really helpful.
Maybe also consider what his GCSE results might tell you about how he will do at A level. Bath and Bristol have a standard offer of ÅAA for mech eng, and I guess most DC who achieve that would average higher than 5 at GCSE iykwim. Maybe he will be fine though, if he got 8/9 in maths, physics and whatever else he would do for A level, and only failed irrelevant ones that he wasn't interested in?
I did AS levels as 1st year of A levels which could go down on the UCAS form which would buoy things up if they did well, but am I right in thinking these don't happen now? (the college only talks of A levels)
Wales and NI still have AS but most schools in England have stopped doing them. So predicted grades will be the key thing. If he can ace Y12 and get strong predicted grades, his GCSE profile will matter less.