Probably a bottle of champagne. If you are particularly close and always done gifts on birthdays maybe £50 voucher maybe one of those that cover several shops and restaurants.
I would think £40 cash or voucher and small present, chocs or booze and a nice card. A bit difficult to call, you could give £40 from each of you ie £80 plus small present and card(s)if you feel he warrants more as you are both god parents.
Do you normally give him a birthday gift? You could double that if you want to make it more special, I would probably think £50. Sounds a nice amount, perhaps a bottle of wine to go with it.