I got married in my country of birth(South Africa) , to an US citizen. I found out that he never registered our marriage here in the US (Michigan).
COuld this be a problem and is the marriage still valid in the US?
In general if a marriage is legal in the country/state it is performed in, it will be recognized without you having to take further action in Michigan. There are exceptions for things like same sex marriages, but provided that is not the case, your marriage does not need to be 'registered' to be valid.