i have the same problem aswell. With patch 1.3 beta initially i couldn't see the inventory of a companion that i took for a quest to train him within 200 days. I could see him in the party, but i couldn't see his inventory. It wasn't there. Then at some point i noticed that one of my main companions was on HOLDING status and i ve searched about that if its a bug. I found out that lot of people have this problem since release and some work arounds and mods can remove the holding status, but still you can't see this companion inventory. He can be in the party, you can't see his inventory or send him to other quests. Also any other temporary companion you take for quests such as family feud quests , you can modify his character sheet, but you have no access to their inventory either. This problem started since 1.3 if i remember well.
Kicking him and recruit him again doesn't solve the problem that you can't see his inventory. You can also use the holding fix mod or developers console mod for kicking and having him back to your party, but still he is not in the inventory tab.