Link Sub-Account PS Vita to Main Account on PS4

I have a master account on my PS4 with PS Now active.  My son has a PS Vita linked to his sub-account of my master account.  How do I download the PS Plus games (free of discounted) to his PS Vita?  When I'm in the PS Store in my main account, his PS Vita does not show up as a downloadable device or system.  When he's logged into the PS Store, he can see and download to his device, but the PS Now discount for games doesn't show up.  Is there a way to link my PS Now account to his sub-account so he can download to his PS Vita?