I'm no SSIS expert but chances are he created the package using the default SSIS protection level setting "encrypt sensitive with user key". This setting ties the package directly to the user account so if his account is no longer active then the package won't work.
To fix this I suggest converting the package to "don't save sensitive" and then implementing a package configuration. However, if that is out of your comfort zone I think you could also convert the protection level to "encrypt sensitive with password" and then specify a custom password instead. This option requires the password to be supplied when the package is executed so it's tricky (to say the least) to automate packages but it should at least allow you to run it manually.
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