Palm Blast

Something taken from Wing Chun and used in JKD is a "Straight Blast," AKA Chain Punching, Battle Punch. Essentially, one strike is coming straight down your centerline to the opponent and as it withdraws in a slightly curving motion downward, the other punch is coming forward to replace it. Think of an uppercase "D" laying on its back. It may look cartoonish, but it can be very effective. In WC, it is often used to enter, but Bruce Lee found it could be too easily evaded as an entry and it came to to the following generations as a good follow-up.

In JKD and Wing Chun, hand conditioning is part of the training. Years ago when I noticed scarring on Dan Inosanto's fore knuckles I assumed that it was from conditioning that he did for Kali empty hands where he would use those knuckles to attack incoming limbs, but when I asked him about it, he told me "No, Bruce made me condition those knuckles for the chop chuie strike to the low ribs." Hard core. 

But many/most people do condition their hands, and in that case I think doing a straight blast with palms is a good alternative as you don't have to be as concerned about what surface you are hitting: if he starts to turn away, palm blasting the side or back of the head will still feel comfortable to and unconditioned hand.