Reverting back to hinduism from islam after marriage

Both my husband and myself were Hindus before marriage. I converted to Islam after more than a year's thought. My parents then arranged a marriage to my Hindu cousin. I agreed but told only him of my conversion, and that he would also need to convert. I felt at the time that were he to convert that Allah would be proud. He did convert and we got married, initially by Hindu ceremony arranged by our parents, then secretly an Islamic marriage. He embraced Islam for the first year. His mother was invited to come and live with us in the UK for 6 months. Once she had left, he suddenly seemed to have reverted back to Hinduism. I am not sure whether this was because he has been blackmailed or out of his own choice. I am also pregnant. He is otherwise a dear and loving husband who is kind and sincere to me as well as to his parents, who do not like Islam. He is also under a lot of pressure with exams and work, and his father recently had a heart attack. He is currently living in my house with my family. Our exposure to Islam from the outside world has also become limited possibly due to pressures on both sides.He probably hasn't had so much of an understanding of Islam as I have had through my brother, also a convert. I can find so many excuses as to why he may have reverted, but what do I do? Our marriage would otherwise be a happy one. Am I committing sin by staying with him? Can I give him time to understand Islam better and to convert back? Please help.