Why not look at Norwegian Cruise lines its Freestyle all the way and you can book the main diningroom for the dress up nights. If you did want to use the main diningroom every night you have to book each day which is the only hassle but you can go from Southampton if you see a cruise you all fancy. Others will respond to your request with different options.
Sorry no one else has so far contributed but you could also look at Thomson's I think it is the Celebration that gets the best reviews but have a look on this site for the reviews and it is also an appropriate name and likely to be cheaper. You could call the number above its FREE and speak to a specialist cruise consultant.
We have been with NCl previously and last year we took NCL Freestyle. Sorry but for us and our friends it was dreadful.The entertainment was rubbish. food nasty unless you paid extra $15-20 each per night to eat in the special restarants .Nothing going on in the day, sports equipment was nearly all broken. We have been on about 36 cruises that was the worse. We are also having our 40th Anniversary this year and we have booked a P & O cruise. They go out of Southampton,your brother will enjoy it. Not too many formal nights. Entertainment is good. We also went on Celebrity & RCL from England with no flights. We had great fun especially in the piano bar at night. RCL have great carnival nights, plenty of activities in the day too. Spoil him go on a proper cruise holiday, you wont be able to stop him once he starts. Good Luck.