Standing to do what? To appeal his sentence, yes, but his sworn admission that he knowingly violated legal orders is binding against him on appeal, so the issues he can raise in that appeal are strictly limited. If you think this somehow now gives him standing to sue in a civilian court to have Obama's eligibility determined, no it doesn't.
Technically it does because Lakin can now show peculiarized harm from Obama's failure to answer the questions regarding his constitutional eligibility.