Did he receive a ban upon leaving the UK?
If he did not receive a ban then the overstay will not be grounds for refusal in and of itself.
The UKBA are not permitted to use overstays as a reason for refusal alone (immigration rule 320). This means that if your husband's application for a spouses visa is otherwise correct and he meets the eligibility criteria then you will be successful.
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If he attempts to come back to the UK on a visitor's visa in the normal way US citizens travel to the UK then they will be able to refuse him entry.
If he applies for a spouses visa prior to travel in order to come her to settle permanently with you (as man and wife) then they will not be able to refuse him entry on the basis of his overstay alone.
Provided he applies for a spouses visa and meets the eligibility criteria then he will be fine, but on a visitor's visa there is a very high chance he will be refused entry.
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