If this were remotely the fact, he would be in jail with the other suspects. He has already been interviewed by the police twice. In addition, I find it extremely unlikely that he would have invited Mrs. Holloway in for a chat if he were involved in any way.
People on this thread can speculate as the might, but note that: (1) he did not lawyer-up; and (2) invited Mrs. Holloway in for a lengthy chat. If he were guilty of anything, why would he take the chance of contradicting what he told the police? He wouldn't.
Thank you for this moment of clarity.
Also, the father's demeanor was incredibly calm and polite. The was he quietly said, "Please come in, please" sounded very humble to me.