Instead of just saying that you need more alone time... try making a spin on the request... so it doesn't seem like, "I need more time with you". Figure out something that you both enjoy doing... going out to eat, movies, biking, hiking... whatever it is and ask him if her would be interested in going with you OR the two of your going together.
I would definitely be the one to come up with an idea, ask him and then follow through with the necessary steps like choosing a date, time etc. Then follow through with it.
Hopefully, this way he won't feel overwhelmed and you won't feel like a nag; but you two will be able to spend some fun, quality time together!