Don't go to the start with him. All the official info says this for good reason (50000 people all trying to get to the same place in a short time window is bad enough without throwing other people into the mix).
My parents saw me 3 times this year. Once at around 7 miles, again at about 10 (it's v easy to get between these points as a spectator), and then on the Embankment at around 24 miles.
I didn't see them the first 2 times as I didn't know what side of the road they were on so couldn't easily look for them. They did see me! I saw them the last time though.
I saw another friend just before 13 miles (she was just after Tower Bridge), and she then crossed the road and I saw her again at around 21 miles.
It's easy for spectators to spot people as the tracking app is really good. Much harder for runners to pick people out.
I didn't run with my phone, but I know some people text their runner to say 'we're on the left just after the 9 mile marker' etc, but I just didn't want to faff around with messages.
At the end it's easy to meet up. You don't need to take stuff for him though- he hands in a bag at the start which he collects at the finish. Put all the essentials in there - flip flops, snacks, warm layer, wipes, whatever he thinks he needs (it's a decent size bag). He then makes his way to the end of the mall (no spectators allowed in the finish area) and there are meeting points in Horseguards' Parade. You just agree which letter you're meeting by. Do NOT rely on a phone signal at the end. You may well not get one. So many people fall foul of this. If you agree a letter to meet by, you'll be fine.