When your dh gets his starters pack when he registers (do you still have to register the day before - I had to when I did it nearly 20 years ago) it will include details of tranport arrangements to the start.
I can remember going to Vicotria and getting the train out (can't remember where to) and then there were shuttle buses to the start. It's all very well organised - but as others have said, make sure he gets there plenty of time in advance, and I mean plenty.
There are so many people converging on one point, if something goes wrong, it can casue a tremendous logjam.
I would recommend not going with him to the start but having amook at the route and working out which places you reckon you can get to - you should be able to spectate at at least 2 places. Calauclate his running pace and allow for the fact that it will probably take him at least half an hour to cross the starting line. Of course, nowadays there are mobile phones (back thne, they weren't common and/or were heavy bricks) which make it easier to keep in touch - don't know if your dh is planning on running with one (personally, it wold be a distraction for me).
You may see him and he not see you or vice versa - but don't worry about it. If you have organised it in advance, then he'll know roughly where to look out for you.
If you have a pushchair, be prepared to curse London Transport for their user-unfriendly stations - although people will probably help you. One year I was supporting a friend and her dh and I had to navigate the Underground with her toddler, still in a pushchair.
You're prbably best buying a one-day travel card, to allow to try to specate at as many points as possible and then to get back to the hotel.