What chickydoo said.
If you really really really have to:
Try and book a bulkhead seat. Even if baby is too big for cot, the extra leg room will come in very handy as you can sit / stand him on the floor in front of you.
If you can't get a bulkhead seat, ask very very very nicely at check-in whether there are any seats next to spares that you could swap into so that you can have a bit more space. The worst that they can say is no. One of our flights we were given two rows of three at the back of the plane - amazing!
Bottles etc - take more than you need, you will be made to taste a couple of random pots / bottles at security, you can then bin these / use them first. Smile and ask very nicely for extra water, juice, rolls etc - your stewards will be more than happy to assist you in keeping your baby quiet IF YOU ARE POLITE. Definitely give a bottle or cup on takeoff.
Calpol comes in individual sachets - useful if your baby has a cold / to help with painful pressure in ears.
Remember a change of clothes for baby.
Remember a change of clothes for you (a jersey dress or similar that rolls up tiny - hopefully you'll not need it, but having spent the last five hours in transit covered in shit coming back from Oz I would never travel without one again...)
ELC do a magnetic drawing pad with the stylus attached on a piece of string. Other winners included - a couple of favourite books and a couple of new pop-up / flap books. Don't bother with anything that can drop and then roll off towards the other end of the plane.
Can you practice watching TV / listening to music with headphones? This was a real issue for my DS (1.5 at the time) as he had never used them before - he would watch the telly for a few minutes, push them off his head, then wonder why Barney the Dinosaur had stopped talking to him and start whimpering... VERY ANNOYING.
If your DS IS walking by then, try everything you can to stop him wandering up and down the aisle annoying people. You will need their tolerance later when he vomits / poos / screams.
If it's a night flight, try and go swimming or something the afternoon before, to ensure he is properly knackered when you get on the plane. Better chance of sleep!
Good luck. And remember - 13 hours is not actually that bad. Is only one long, awful day / night.