If you have good Lut Sao Te Chung, you should be hitting him first. If you are hitting simultaneously, you need more Lut Sao De Jung, you really need to pry from the elbow and hips. The opponent should be afraid to let go and go round, as he will be hit immediately.
As someone else mentioned, dont just tap him, all hits should have the body behind him. If he is slapping you, whilst you are stepping forward and driving his chin or chest a few metres back, he will soon get the idea who will come off worse.
To be honest, he is kidding himself in training a move which results in a you getting off a simultaneous counter. Even if he hits first, your hit will do more damage. And does he really think a slap is going to work in a real fight?
If you can’t hit him first, no matter how good your Lut Sao Te Chung, then his arms are too long, so you need to angle as you go in, turning into his strike, maybe turning your punch to tan, then, when you have the distance, turning it back to a punch.
I appreciate what SauLauChung says, you dont want to over-rely on Lut Sao De Chung to the point where you are blindly pushing forward, and thinking that solves everything. Thats why the pry comes from the elbows and stance rather than brute force in the arms and shoulders. But there is also a danger in forgetting that the direct, and simple ways are often the best. Why move, pak, bong or anything else, when a straight punch will do the job?
Next time he does it, take him off his feet, and continue to do so until he gets the message that it is a dangerous thing to do.
Good luck!