How do you quit a job after one week?
Avoid telling this manager you prefer your other job. Heaven forbid you want to return the manager may be resentful and not hire you back if you needed to return.
Tell him you are very sorry, but you received this job opportunity that you really want to pursue. Not to take it personal because you sincerely like him and your co-workers, but to please understand if you didn’t take advantage of this opportunity you may be left with the “what if’s? If he is an older person he will certainly understand.
Someone young (young meaning 20′s) they will still be too wet behind the ears to understand. But DO NOT say you like the other place better.