The only way to get over that feeling of anxiety around initiating is just to start doing it, in my experience.
If you have a fear of rejection, it's horrible thinking he might say "nah". But equally, he has to face that fear every time he initiates, right? And if you say "no", it doesn't mean you don't love him or fancy him, just that you don't want it right there and then, yes?
Same goes for you. If he says "I'm too tired" one time, it's not the end of your relationship. It just means he's too tired.
As to how to do it - I'd suggest if you're in bed, put your arm round him, rub your hand over his chest, kiss his shoulder or neck and say "Fancy a bit of fun?" or "Are you up for it?" or whatever phrase you'd usually use. Then assuming he says yes, start the foreplay. I'd usually grab his cock and then start a blowjob. Usually gets things moving nicely 