All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Also, he is not permitted to declare that the child is not his, and the child is attributed to him in Islam. It is only through Li’aan that he can declare disownership of it. For more benefit, please refer to Fatwa 90098.
Also, he has no right to accuse you and your family of plotting against him without evidence. On the other hand, if your assumption that his other wife brainwashed him and is trying to push him to divorce you is not based on solid evidence, then it is a negative assumption which Islam forbids; Allaah Almighty says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin...} [Quran 49:12]
Divorcing a pregnant woman is valid in Sunnah. So, if a husband divorces his wife while she is pregnant, the divorce is effectuated. For more benefit, please refer to Fatwa 82929.
However, keep in mind that a wife is not entitled to ask her husband for divorce except for a sound reason, and him marrying another wife is not a sound reason. For more benefit on the sound reasons for asking for divorce, please refer to Fatwa 131953.
Allah Knows best.