Meet him at the finish line?
No, seriously, it depends on your goals and priorities. If you want to run the best time, then just plan to run together as long as you both want to go the same pace, but don't worry if you separate later. Don't feel bad and spend energy worrying about it if you separate. If it's more important to you to run with him the whole way no matter the pace (for support, encouragment, friendship, etc), then just tell him those are your intentions.
One thing about running with someone the whole way- it results in the slowest time. The reason is that, if you feel good and he feels bad, you run at his slower pace. But then if he starts to feel better and you feel bad, he will run at your slower pace. So you run at the slowest pace the whole way. Unless I'm pacing someone, I never plan to run with them the whole way. If you happen to run a lot together, great. If not, you'll see each other at the finish. But I'm always trying to run the best time rather than run with friends, so your situation may be different.
In the mean time, you may want to figure out if he just happened to have a bad race/bad day, or if his disappointing half-marathon is really a reflection of lower fitness. This may make a difference in your marathon plans.