I'm going to be straight up here. Was it "accidently" on purpose. In the hope he wouldn't go? Or would defer.
Your post sounds slightly immature, you have obviously been arguing over his career choice and are only just starting to come around to it. You don't even know yet if your relationship will survive the "test".
He does deserve to know absolutely. This is however a massive career oppertuntiy if you are trying to force his had, it won't work. Wiser people can tell you that. Trying to stop your loved one progressing will fuel resentment. It will come full circle and bite you on the bum later. If he feels pushed into giving this up now, or trapped, then it could well cause issues later. In fact I gte it.
Before people jump one me, I do know it's both the partners responsibility etc, but I'm replying as I see this post. TBH sometimes people need reality spelling out.
He has the oppertunity of a life time, I think you need to be clear, with what you expect, and what you expect of him and for him to do. If he goes can you cope? Can you live by yourself? You don't know if you will make it as a couple after training, what happens with a baby too? If he gives it all up for you and the baby. How do you think he's going to feel about that down the line? How do you think it will affect your relationship? What will he do for money or a career if he gives this up? When do you think regrets, if any might start creeping in?
Basic training and trade training is only the start, welcome to a life of basically being a single parent. Having to suck it up and get on. It's fun but my god a difficult life.
I wouldn't want to be a fresher "wife" whilst having to deal with adjusting to motherhood in all honesty. Plus as a dp, not a dw, you won't get much help from the forces, or many concessions, wrt, leave, help, support, compassionate leave.
I'd think very carefully about what you are going to expect from here on in. If you don't even think you will survive the test on your relationship when he joins, you are going to have a shock with a baby too. If he does give it all up as he feels he has too, you may also find your relationship deteriorates.
So yes tell him, but you need to think very carefully about the possible outcomes.