I'm at your beck and call.
He's at my beck and call.
I'm there for you.
He's there for me.
I'm/He's ready and able.
I'm/He's ready and waiting.
I've got you covered. He's got me covered.
I've got your back. He's got my back.
estar alguien
al ~ del cañón.
1. loc. verb. coloq. No desatender ni por un momento un deber, una ocupación, etc.
www.rae.es
He's always at it. He keeps at it.
He never rests.
He's a workaholic.
He keeps on it.