Successfully dealing with the sensitive issue that occurs when one partner is not "in the mood" for sex, for whatever reason, can be a very important, positive step in your relationship. Failed sexual expectations can be quite damaging even if such hopes are unrealistic. And, you cannot be expected to always be ready for sex just because he is.
It is very important that the two of you communicate here by talking, and in such a fashion that it does not become an arguement. Let him know that you love and really enjoy sex with him, and that it's great for the relationship. But, there are times, for instance when you are tired or when youv'e had a very busy day, that all you would like to do is go to bed and have a cuddle. Would that be ok with you?
Good luck with such a discussion!