Marital status is not an "eligibility factor" with Medicaid or WIC, so it doesn't matter.
All that matters, is the number of people in your household, and total household income.
If you DO get married and he can add you on to his insurance, you may or may not still qualify for Medicaid, depending on your household income and which state you live in (each state sets their own rules and eligibility for Medicaid).
If you lied about living with him, and kept his income off the application, to qualify, then if/when you do get married and add his income in, it might knock you out of eligibility, might not. If you were honest about living with him, the marriage won't affect the benefits.