i have a 6 month old silkie cockerel and he is living in with the rabbit. I put two bowls in the pen one with rabbit food and one with chicken food but the cockerel will only eat the rabbit food. if this safe? and how do know if my cockerel is drinking enough? below is a picture of the rabbit food.
If he has access to water then he will drink what he needs just like you. Simply put, he is not going to get everything he needs from rabbit food. The rabbit food wont directly harm him but it will be deficient in the nutritional needs of chickens and that will be the problem.
Your rooster also needs other chickens. If this a very temporary situation, no harm done. But I would put him with other chickens as soon as you can, both for social and nutritional reasons.