If you only go up to a 5, you can't really add any tank mates. Tank mates aren't suggested in anything less than 10 gallons. However, upgrading is totally up to you. Your boy has a very nice 2.5, but of course he wouldn't mind a 5 either. I'd say if you have the money and want to do it, why not? Lol and then you have an empty 2.5 for another friend...! 
