Can you prove he was?
Sadly, the burden of proof is on the Muslims for these kind of things, not the non-Muslims. If one is going to claim that the first person to exist was Muslim, they should have proof for it. If they're going to say all children are born Muslim (despite children not doing anything Muslim-y), proof is needed.
There is little proof needed for 'children are not born Muslim', because I have children and many who do not raise them with religion do not go to Islam, and it's a case of logic that Adam could not have been a Muslim.
I don't mean the whole 'Muslim means submitter to God, and he submitted to God, so he was a Muslim' thing. That's Semantics, and it just makes the argument look incredibly weak. We all know that Adam would have been one who submitted to God, as well as all prophets would have been submitters to God (otherwise they would not have been prophets now, would they?) - but it doesn't make them Muslim in akin to the modern concept of Muslims.
There is a common identification with Islam today which logically we know that Adam would not have been able to do. Adam would not have been able to perform Hajj to the Kaaba, he would not have been able to declare that there is only one God and Muhammad is his messenger, he would not have been able to say the Qur'an is the holy book of God... So could Adam have been a Muslim akin to modern Muslims? I doubt it very much.