Lol Tinker, my 5 yr old dd does this too at bedtime..."my legs are too floppy" and wants to be carried to bed!
With the dressing thing Tigermoth, I think tufty's advice was great. Any praise for any effort is the best thing, rather than pyjama threats and belittling comments about not being a big boy etc., which makes dressing seem a negative experience.
I have the same problem with my dawdling dd mind you, and the only thing that seems to work with her is to turn off the tv and to tell her if she is dressed and ready for school by 8.15 she will have time to draw or colour in (her favourite things to do at the moment!) before we go. Maybe if your ds has a favourite game or pastime you can offer that as a reward for being ready (as opposed to a punishment by denial if he's not!) In other words make sure he's aware beforehand what he could get for cooperating, rather than telling him afterwards that it is too late.
If your mornings are hectic as most mum's morning are, maybe he is just looking for some one on one time with you and he thinks he has hit on a solution! Even though it might be mum ranting or being cross (been there, done that), it's still mum's full attention, as tufty suggested...Is there any way you can give him a different kind of attention, arrange things so you sit and have breakfast together and talk, or maybe your dh could dress your other son instead of you, and you could take over breakfast or teeth? I know mornings can be a nightmare but it sounds like it might be worth trying a change in routine to see if it will help!
The only other thing I would say is to ask his teacher if he has any fine motor skill difficulty, eg writing, threading, cutting, tracing etc. As he is still not dressing himself in front of his friends at school maybe there is a deeper problem, but it would be worth finding out if dressing is the extent of it.
If he is otherwise fine then it is probably just attention seeking or simply dependence on you as his "dresser"! Children should be largely self-dressing by age 3 or 4...if he is used to you doing it for him maybe he's just never bothered to learn. I know it seems easier to do it yourself when you are in a hurry, but in the end they think they are unable to do it themselves.
So basically I think the encouragement for any effort and a chart would be a really good start; also to check with his teacher on any other aspects...let us know how you go!