i want to assure you that you are not teaching him that he needs to nurse when going to sleep. it might be a long time before he does go to sleep without nursing, but it will end.
it all depends on what you want to do. it sounds like you are doing whats right for you and your baby by tending to his needs! keep it up! you can start giving water in a cup around 6 or 7 months and see if its just a drink thing, using a pacifier, much more reliable than the thumb, and easier to get rid of later!
i will tell you my story. i nursed my son until JUST recently when he weaned himself - he is almost 20 months old. he nursed a LOT while going to sleep, and would not sleep in his crib AT ALL until around or after a year old. i figured it was nice that he would accept no substitute for good old mom and dad :D!
i will be the first to tell you it is exhausting! i will also be the first to tell you that i wouldnt change a thing! my son has, for about 3 months, been able to be laid down in his crib, and he will go right to sleep, no fuss, no crying, no tantrum, and he will sleep through the night. he does wake up sometimes around 4-6 am and wants to be with us, especially lately since hes been sick, but he has many times now just slept in his bed, or been able to be laid back down in his bed after a drink of water or something.
the important thing to note, mom, is that you are doing the right thing by responding to your son, and doing the things that will help him to grow in a healthy attachment relationship! this will be important later when he goes to day care, because he will judge your reaction strongly when being dropped off.
make sure you can go there a couple times before actually leaving him there for care. watch how he reacts. im going to assume as an attached baby, he will watch your reaction, go play a little, come back to you, go play a little... this is a GOOD sign, a sign that he trusts you and he wants to come back to you to 'touch base' with mom. most kids just run off from mom and dont even notice when she leaves, or they cling so strongly that mom feels horrible leaving. if you are securly and healthily attached, baby will trust you and your leaving him there, that you will return, and he is safe.
i know it was a little off the topic... but i am really encouraged that you are having these feelings about responding to your son. heres another suggestion. buy a moby wrap! they are extremely comfortable compared to other slings i have used, and even though they range around 50$, they are money WELL spent. i got mine when my son was 16 months old, so even though i can still use it, i was sad that i hadnt bought one before! i use it for a baby in my day care that refuses to let me put him down. it is awesome! the minute i get him all wrapped up in it, he stops crying and is either in that quiet observation mood, or he goes right to sleep. this will help you be able to do things while baby sleeps, after all, he spent 9 months inside your womb being rocked by your motion, its most natural to want to be like that after hes born too! and the more you can honor and respect those feelings, the more securly your baby will trust and attach to you.
trust me, you wont regret it - the hard work you put in now will help you in the future! :D\
any questions just ask!