Why not just blast him with jabs and rear crosses as he comes at you, if you get overwhelmed and need to block I would use covering blocks where you bring up your elbow high toward your face, move in quick and clinch with him using a double overhooks clinch, it works for me. If you just jab the crap out of his face and thben throw some crosses he will be out pretty soon, your puches will land sooner than his and will be more effective. Try that. Whatever you do avoid moving back if you can because he will just keep pursuing you and overwhelm you worse.

Is he taller than you?

Duck under it, square off with him and go
- then unleash with every fast, direct punch you can possibly land. Continue until opponent drops, or gets one in on you.

Stab him

Defence Defence Defence. Cover up and move away. He will get exhausted. This principle applies to every martial sport. Key is to know how to ride the storm. Defence, always defence.

Oh and you can keep them at long range with hard front kicks, side kicks and round kicks.

I assume you are boxing without kicks.

Try changing the way you hold yer hands if you are getting hit by hooks/haymakers. The traditional MT style "peekaboo" hand posistion allows you to protect your head very well from both kicks and hooks/elbows etc.
It is fairly easy to slip one hand back over the temple area in time, this will block most hay makers and even good hooks. (for kicks it is best to use both hands if possible, one at the temple one to parry. Then you can parry or even trap the kick as well. For protection from elbows the forearms cross horizontialy over the face )

Work on slipping, even just a simple turn of the head will allow you to take or slip the punch and stay inside too counter punch. Slipping and ducking the punch and hitting the body might work, a good body shot can K.O.

Parrying a haymaker is very effective, the guy likely is already off balance from too much torque any way, a little extra push might spin him right around even giveing you his back or causeing him to fall.

(in MT the classic parry is usually done vs a jab, then followed instantly by a very good clean leg kick, or even a head kick that is very hard to see comeing as it strikes the rear of the head)