Ahhh, he's experiencing the first throes of adolescence! The rush of testosterone makes cockerels (young roosters) really interested in sex. The grab is to hold the pullet/hen so he can mount her.
The clucking is called "tid-bitting" and is his call to hens for a goodie he found for anybody who will come running. Sometimes they cheat and fake the girls put with nothing more than a small stick, or a leaf, or a pebble, not a juicy bug or other edible. Treat. This is to get them close enough to grab to have his way with her.
The nesting behavior is also instinctual, and is his way of showing his hens good places to lay eggs (to hatch and carry his genes forward).
It takes about two months or so for him to get his groove on and be respectful to his hens; in the meantime, he's just wanting some, if you know what I mean.
I've had bantam roosters mount large fowl hens and they can be successful, but usually it's something that just looks funny and makes me laugh. It ticks off senior hens, too.