I'd be calling an ambulance if you can't get a car - at the very least, the paramedics will check him out at your home and determine whether it's worth taking him to the hospital. That way, if it's not anything potentially life-threatening, you can still get him checked out without the hassle of organising how to get home (eg if you get there by ambulance).
I wouldn't hesitate to call the ambos - when it comes to children, better safe than sorry, I reckon. It's not like it's a stubbed toe or a grazed knee, kwim?