Firstly, have the college definitely accepted that he is entitled to a small room? There is quite strict guidance on this now, and I believe medical evidence is needed. Some schools do bend the rules on this so it may not be the case he is entitled.
Secondly, I believe the "small room" applies to the number of students in a room, rather than the actual size of the room. "Small room" is not the same as being alone in the room, so the college may have technically met this.
Specific seating is a separate access arrangement. If he needs to be sat away from certain parts of the room, this can normally be easily arranged BUT it is a separate access arrangement and would need to go through a formal process.
I fully understand it is not ideal, but JCQ have really tightened up a lot of the rules in recent years, and if a centre is found to not be following the rules correctly it can have major consequences for everyone.
So you need to decide what it is you need to request- e.g. solo room, specific seating etc and contact the SENCo/exams office and see what evidence they will need.
Don't forget, with a btec exam he can retake in the summer if he doesn't get the grade he needs.
Did he get his extra time?