you can ask him as you are both partners and should be aware of the income that comes in to the family. this will help both of you plan well and save too. talk to him nicely, in such a way that he doesn't feel you are questioning him. tell him that you want to know because you are interested in saving for the home and also budget accordingly.
it all depends on the kind of relationship you share with your partner. you can ask him about his earnings, but i wouldn't say it should be in an interrogating manner. there are many couples who do not share every detail of their income with their respective partner, but have a joint pool for family expenses and savings.
Of course, you should ask. You are one in term of not only physically but also, emotionally and mentally, so why not. You are not the 3rd person in your house to hesitate, you are his first partner in everything so he should tell you everything...
i think dat u shud participate in all financial matters.. atleast u shud know wat amount of money is being invested n whr...both of u shud b aware of each n everything..