Hi fuzzpig
My DS2 is 2.4. His speech is a little ahead of your DS's (he has made a lot of progress in the last couple of months), but it's not great and certainly far behind my DS1 and DD at the same age. I also find the same as you - that he is more prone to tantrums than they were, and less easy to reason with.
Like you, I have a tendency to assume that his understanding is behind theirs, as well as his speech. However, I'm not sure that is true and I think it's something we need to be careful with, as you can end up excusing bad behaviour on the basis that 'he's too little to understand'. After all, even if his speech and understanding is poorer than theirs, and even if he has hearing problems (worth checking out of course), he should still understand the word 'NO', spoken clearly and accompanied by a shake of the head, shouldn't he?
My DS2 is fiercely independent, much more so than my elder two were. Often his tantrums occur because he wants to put on his coat by himself (or similar), and I'm too busy pulling it on him to realise that's what he wants. Because his speech is poor, I have to take the time to kneel down next to him and figure out what he is trying to tell me by a mixture of words and actions. He is very determined, but I definitely see an improvement in his behaviour if he feels I am trying to listen to him and communicate with him.
Different children do respond differently to discipline methods (eg some children respond better to a reward chart than the naughty step), so I think you can't assume that what worked for your DD will work for your DS. You may have to be creative in coming up with some new disciplining techniques and ideas. The golden rules stay the same though - stay calm, be consistent, follow through on warnings.
My DS2 is much harder work than DS1 or DD, so I do sympathise with you. I try to see his good points though - he is very affectionate and sociable - after all, it would be boring if they were all the same!