I think you might get better answers if you provided more info. What do you mean it claims to be his first autograph? Did you buy it from a retailer with a certificate of authenticity? If so, is the retailer legit? Is the authenticator legit?
It's not going to be worth five figures. It's not likely to be worth anything tbh. You could always take a pic & tweet it at him, see what he says. That would be the best way. I had Mark Gross authentic his autograph for me via twitter.
Next, if you're worried about it, I'd suggest you post it on Reddit.
Otherwise, like I said before, there are authenticator services but they'll cost you more than the autograph is likely worth and they're still not 100%, more for if you sell for buyers to have peace of mind that it's real, but this likely won't be worth it.