Break it down for him into a manageable chunk, like
"look its only 20 weeks / 65 days. "
Then remind them what they'll be out in time for ie
"You'll be out to celebrate my birthday /Xmas" etc
Tell them you will visit & write regularly & keep them up to date with everything. Plan things to do once hes out - trips, parties, anything that he can focus on. Also do some research into places that would be willing to employ him when hes out so he's not panicking about survival.
Remind him "look, once you've served your XX days, its done & u never have to do it again".