someone posted a link a few weeks ago for speeches to do with step-dad/father-of-the-bride/best man I think it was etiher Andy or Chris. Will see what I can find.
As he is the best man he must be a good friend/relative and obviously highly loved/respected by the groom to be. He could write down all the memorable moment that they spent together, all the ups and downs during their frienship, all the mischief they got up to etc etc. Then pick out the good bits, funny bits (leave out the naughty bits). All the best mans speeches that I have ever heard are along these lines and seem to go down very well.