Felix is too catfood.
If
Teddy was, say,
Edward or
Edmund or
Edwin, I'd consider it happily. But not just
Teddy.
If someone with either of the Irish names ever left
Ireland, he'd have to resign himself to a lifetime of explanation, or to a name change.
That seems to leave
Leo and
Blaise, both of which I like. But
Leo is a bit trendy, so IRL I'd go for one of the full forms of
Teddy.