Imagine being fourteen and your mother having a new partner plus a baby on the way.
Then remember that at that age, the brain is wired differently - aka puberty - and you are at a rather vulnerable and insecure stage of life. (But would probably rather chew up your tongue than actually admit it!)
The baby bump is the very obvious proof of a few things.
- Your mother had sex. Eeeeeewwwww!!! You mom used to be the person that took care of you when you were sick, cooked you breakfast and all that. Not an adult with a sex life. Plus, isn't she too old for that? ( For most 14-yos, everyone beyond 25 is "old".)
- There is a new baby on the way. This might replace you, kick you out. First the new partner, now the new baby. What role will be left for you? You are from the old stage of life, from a past relationship. And certainly not able to compete with a cute baby.
- And that baby will cry, poop, take up all the free time of Mom and the family and mean the end to all the grownup fun you were just about to be old enough for. Instead, the family will now be dragging that drooling little monster around. You might even be forced to babysit! Your buddies will laugh themselves silly!
- You had absolutely no control in that case. You weren't involved in any discussion about having a baby or not and likely not involved in the search for a new partner (Substitute dad?)
And now the whole town will know! They will all look at him! This is sooooo embarrassing! So let's try to gain at least some control back by forbidding Mom to parade around with that baby bump.....
Yes, I know that this is not logical. You and your partner probably told him that nothing will replace him and that you will love him just like always, that the new baby will be an addition, something to be happy about, a blessing.... Unfortunately, logic has its limits with 14yos. I'm afraid that your teen will just have to accept the situation. But of course that doesn't mean you can do nothing at all.
- Do grownup things with him - perhaps "boys stuff". He will need an ally and a role model. And once the baby arrives, your partner will have less time. So if you two have a good relationship, it'll be good for him.
- Tolerate bad behaviour to a degree (there will be a lot of inner tension and insecurities, see above), but also set clear boundaries.
- Listen, be open for conversations. If you can get him to voice his insecurities, good. If not, accept it. But stay approachable.
- Show him love. Not just tell him, but find ways to show you love and accept him. I know that male teens and expressing love can be almost mutually exclusive and many grownup men are challenged by the idea, but I'm sure you will find a way. Your way.