Hi there, I am a Reception teacher and what you describe OP is quite a common thing, particularly for boys. From what you say about his writing at the moment, I would say that there is nothing to worry about. Children at this age are learning to blend sounds together to make words and learning words like I, no, go, to etc. so he sounds like he is on the right track. If he were my son (mine is 20 months so got all this to come!) then I would do as others have recommended and talk with his teacher if you continue to be concerned.
Katykuns, I would say that expecting children to be able to write their name by the end of nursery is quite unrealistic.
I understand the worry but in most Scandinavian countries children don't start school until 6 and they perform better in reading and writing. How they do at 3/4/5 does not necessarily show how they will end up! I have taught many children who have started off 'slower' in comparison to their classmates (but I really don't like to compare) and they've all of a sudden, almost overnight, just got really interested and have taken off like a shot.
OP, what is your son interested in? Could maybe gently encourage him by writing about his favourite things, making number plates for his toy cars, writing speeding tickets, making up pretend names for dinosaurs, asking him to write your shopping list or something?
I know it's difficult, but try not to compare to his cousin, your DS will have other skills that his cousin does not have, It's just that reading/writing is seen as a more intellectual thing. Your DS may have great social skills (which in my opinion is more important, particularly at this age). Continue to encourage him and not be pushy as you may turn him off it completely.
Wow, didn't realise my post had got so long!!
In short, don't worry, he sounds normal!