Right outside the house too, as he was opening the door, 5 guys jumped him, armlocked and punched him in the eye
Should I tell him to get some OC spray or what?
Five-to-one with surprise in their favor is anything but fair odds. I doubt that there are any devices that would have help your father at that point. OC spray would have probably just pissed them off unless you carry a fire extinguisher worth of the stuff.
Situational awareness would have been key. Also, keep in mind, that this works both ways. The would be muggers may have targeted someone else if your father looked like a riskier target. Look at his comportment and appearance; whether he has a gun or knife, may be less important then whether he appears he could "possibly" have one and was prepared (read crazy enough) to use it.
I also grew up in Brooklyn, and had my father jumped at night by three guys. In his case it happened as he was walking down 9th street, between 4th and 5th avenues, on his way to a late shift. Two guys ran up from behind and another walked from across the street to cut off his escape route. One of the guys threw an arm around his neck to pull him off balance as his buddy also coming from behind circled to continue the assault.
Fortunately, my father had first noticed them about half a block away. He was walking down a fairly desolate street, so there was no place to go, but he did keep an eye on them. As the first guy's arm came around he elbowed him hard in the ribs (guy one dropped to the ground), then punched the second guy in the face, and turned and pulled a hunting knife on the third (he already knew there were three of them). He surprised them and they took flight immediately.
Just having a knife probably wouldn't have helped; but, being aware of how many there were, how they were going to hit you, how you were going to defend yourself, and, getting ready for it did. Then again, he may have been able to diffuse the whole thing by simply turning around. I still remember taking many late night trips on the old J-line subway; every time you reached into your pocket everyone else would too, and as soon as your hand came up with a roll of Certs there was a nearly audible sigh of relief. (that shifty-eyed little old lady sitting across from me with the knitting needles was probably packing... I was happy to see her hand come up empty from that little knitting basket

)
My best to your father. I hope he gets well soon. Muggings use to be so common in NYC that I use to carry a prepared wallet to drop (a few bucks and papers, expired CCs....).
n2s