Nobody says you have to get it all clear at one time. Take it in baby steps. Give yourself a small area and a small chunk of time.
Childrens items: nobody says you have to save them. Having another child won't mean that it'll be the same gender, or even if it is, that the clothes will fit. Kids are all different shapes. Elastics especially will age, and will go bad even in storage. Give it to some worthy cause. You could halve the amount of toys, since any new child will get more toys just from birthdays and christmas.
Professional and Techie stuff: I know this, I"m both! If he hasn't worked on it in a year, it's going to be too out of date to be worked on. If he's a real techie, he'd want new tech anyway. I have a bunch of books too, but the versions are currently down level, and you'd be better off updating with current material when you do go back to a professional setting, unless it's really foundational stuff or you're looking to get pegged into working with legacy materials. Reference books in this age can get outdated pretty quickly, depending on the field you're in, so let them go be useful to someone before it's too late.
Parents and family material: I can't help there. it's going to be hard. I had a bag of my grandmas clothes that my mom asked me to keep. 10 years later I asked her if she wanted them, and she'd forgotten all about them. So I donated them. Maybe you should try not being secondary storage for packrats.