A. Our boss is ill, so Jack is currently filling in for him.
Feedback When we use phrasal verbs we should remember that some of them are separable while others cannot be separated. 'to fill in for someone' belongs to inseparable phrasal verbs.
B. Our boss is ill, so Jack is currently filling for him in.
C. Our boss is ill, so Jack is currently filling him in for.
D. Our boss is ill, so Jack is currently him filling in for.