I know how you feel, I do not like saying no either, and I do not like making others sad or upset, but, make sure you are happy first before you start worrying and thinking about if anyone else is happy or not. He is more likely to be happy if you are happy as well. If you are happy you will make others happy, if you smile you will make others smile (it works, even when walking around outside, I have tried it). If you are happy with everything you are doing, chances are he will be too.
doing most sexual things will feel enjoyable, esp where your undies are, a lot of what is there is designed for pleasure, and masturbation will help you find out what is the most pleasurable and enjoyable things to do down there, and you can use this to let him know what you enjoy the most. I would suggest that touching him may give him the same feelings as you have when he touches you (just make sure you have some towels
).
I would suggest though that you do not sit naked on his wet pants/undies, not let him get your undies wet, esp if you can feel that they are wet with them still on, and you don't let your undies touch his wet undies, as although it is unlikely, it is still poss for sperm to travel through undies, and if you can feel wet through them, something has passed through them in order that you can feel that they are wet.
on a side note, men can only store so much sperm/seaman at a time, after which they have to ejaculate before they can produce more (I am not sure if that is the right way to explain what I am trying to, but the basic idea is there), and, from personal experience, if a man ejaculates enough times in a short period of time, seaman wont come out afterwards (although I can not guarantee there wont be sperm there), and also, in my experience, he may be able to last longer the second time, and longer the 3rd etc. Just something else you could think about experimenting with.
Imagining is good, esp while masturbating. Wet dreams are even better, as long as you wake up either at the very end, or just after. They are obviously no fun if you don't remember them lol. Waiting while imagining can also make the experience even better than if you do not wait, due to the anticipation, amongst other things.