Gmail will be perfect for that.
In his Gmail account, tell him to follow the following steps:
- Sign in to Gmail.
- Click Settings and select the Accounts and Import tab.
- Under Send mail as, click Add another email address.
- In the 'Email address' field, enter the address from which he will be sending emails (your address)
- Click Next Step >> and then click Send Verification. Gmail will send a verification message to your email address to confirm that he can use it.
Then you will receive the email verification, you just have to approve it.
Warning: Be careful though, I don't think you can remove this privilege from his accounts yourself or force him to remove your email address later on.