All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
What you should do is to help your husband in performing acts of obedience to Allaah. I'tikaaf (to reside in the mosque for the purpose of worship and devotion to Allaah for a limited period) is one of the good deeds by which Allaah praised his slaves as He Says (which means): {[Such niches are] in houses [i.e. mosques] which Allaah has ordered to be raised and that His name be mentioned [i.e. praised] therein; exalting Him within them in the morning and the evenings. [Are] men whom neither commerce nor sale distracts from the remembrance of Allaah.}[Quran 24:36-37]. So if your husband is keen on making I'tikaaf; you are getting the same reward as him, because if one helps in righteousness or guides people to it, he will be rewarded. Allaah Says (which means): {Help you one another in virtue, righteousness and piety.}[Quran 5:2]. So your husband is permitted to do so as long as there is no real harm on you if he is absent from you during the whole night, for instance if you are afraid that someone would sexually abuse you in his absence or fear other real harm to happen to you or to your child. In such a case there is no harm on you to ask him not to make I'tikaaf in the mosque as it is an obligation on him to protect his wife and children but I'tikaaf is only desirable, and an obligation comes in priority over what is desirable.
Allaah Knows best.