My boss’s philosophy is to really push people... in their first 90 days... I am being encouraged to assign a very ambitious workload to my new employee and put some pressure on him to meet an ambitious deadline.
Is this a good idea?
Not in my opinion.
Assuming you already believe you don't have a choice (like talking your boss out of this), I'd say that you are a new manager, and if you want to stay one you don't have much choice than to do what it appears you have been directly ordered to do.
I would tell the new hire, "My boss X really likes to give an opportunity for people to shine in their first 90 days, so I'm assigning you project Y at his recommendation. I will work closely with you and help you. I just want you to know it is a stretch project and not to get discouraged. It will be tough but we think you can do it."
At least that's what I hope I would do - I'm sure you can see some pitfalls in coming clean with the new person (like if they disclosed what you said it might reflect badly on new manager 'Jeff').
