Your older dog is showing dominance over your younger dog by humping him. In dog language, this is considered rather offensive behavior.
While I'm not a proponent of intervening TOO much while they are establishing their relationship; I probably would try and keep the humping to a minimum.
You can separate them immediately when it happens - for a few minutes. Your older dog should get the idea pretty quickly.
__________________  Mommy to Lucy, Ringo, and Matthew |