He will have to imagine the things that his father would have mentioned;
Her good looks;
Her acheivements;
His pride in her and in this day;
Compliment the grooms good choice;
Praise the grooms family;
Instruct the groom to take good care of her.............
He will (naturally) have room to insert 'brotherly' comments, i.e. that she only looks good after spending an hour in the bathroom using the entire contents of a small chemist to achieve it..........but, these amusing asides shouldn't be allowed to over-balance the 'tribute' from absent father to daughter.
If he writes his Dad's speech out first, imagining his Dad's voice and tone, he will find gaps and pauses in which to insert his own 'sniggering' comments, like he would if his Dad were speaking out loud.
Be a sounding board for him.........let him try out every write and re-write until he knows exactly what to say, that way some of the nerves will disappear.