I'd hold down his nose and or forehead with your right hand and open his mouth by pressing lightly down on his gums with your thumb. My horse did you same thing for a while, but as he got used to it, he now lowers his head and excepts the bit willingly.
Make sure when you put the buy in it's very gentle and doesn't hit his teeth. Do the same when you take the bit out. My horse became mouth sour because his previous owner would knock the bit on his teeth. Also make sure if it's cold outside that the bit is not cold. When he throws his head up just be patient and let him put it down on his own so he feels he's not being forced. Also look at your bit it might be he is expecting pain when he is bitted up even if you aren't intentionally hurting him. You may be pulling on his mouth to hard or the bit is too severe.