It is entirely up to you but you are essentially talking yourself into taking a risk with your baby.
A dog that is 'lovely' 70% of the time isn't lovely, it's unpredictable and aggressive.
JRTs are notorious for this and get a bad press. While you're right not to steroetype individual dogs, the fact is that everything you've said indicates that your JRT is like this - snappy, aggressive, not placid and not currently responding at all to whatever training is taking place. If the training makes no difference and effectively you are regularly getting bitten then yes you have a massive problem.
I would rehome for the dog's sake as much as anything else. This is not going to go well - once the baby is here you are just not going to be able to trust him one iota and that means that he just isn't going to be able to be with you and interact with you as he has been used to doing. That's sad for any dog, for one who already sees himself as 'boss' and feels confident with biting and threatening to communicate with you, it's disastrous. The outcome will likely be that he becomes more aggressive because he will be unhappy.
He's only young, you're looking at YEARS of constant barricading/warning/can't leave doors open because of the dog/can't let baby crawl around there/cant really have friends with babies round for coffee morning, the dog will go nuts and what if he gets out...
Miserable for all concerned.
For the dog, I would also say that he's started off so badly for this behaviour to become normal to him that he really needs a fresh start- a new home, new boundaries and definitely no babies. That's his best chance of not becoming one of those horrible little dogs that are just nasty, and often end up PTS for biting.
Horrid for you but I can't see how this is going to work out at all - and, as I said at hte start, it comes down to taking a risk. The JRT that killed that baby jumped a stairgate. It isn't worth it.