First of all, the name Autumn is the most popular season name, albeit for girls only, even though for me it sounds boy-ish. However, there are alternatives for boys if you don't wanna risk it.
You could start by looking up the name Autumn in other languages: Automne, Outono, Ruduo, Syksy, Haust, Podzim, Autunno. All of these sound exotic and fresh, and perfectly usable on a boy.
And if months like May and August can be used as names, why not go with September, October or November? All three are pretty usable as names, especially the latter names, with cute nicknames such as Toby, Tober, Ember, Nove, Novie, Octo, Octie, Sep. None of these month names are popular at all, so they haven't been given a gender yet - go grab them for your boys while the girls haven't put their hands on them.
