It is far, far easier to have the hen raise him. Making a new nest for her in a better location or a cage and putting the baby under her at night may work, unless she rejects him now.
Heating pad is a no-no and will permanently damage his feet/toes.
He will die in a few days without the constant proper temperature of heat lamp, approx 100 degrees in a corner of a safe box, feed and water, and grit. Also a companion or at least a feather duster or he will peep constantly and exhaust himself.
Check backyardchickens forum for loads of great advice, Good luck with him/her