Typically, we find that players between the ages of 10 and 13 are comfortable with a 12" first base mitt. This 2017 Rawlings Gamer Youth Pro Taper 12" First Base Mitt (GFM16B) will feature a tighter wrist opening and slimmer finger stalls that works well for players your son's age.
Although I think your player would be comfortable with this glove, a 12.50" would likely work for him, as long as you found a model with a similar youth fit.
Typically, for an 11 year old player we would recommend getting a 1B mitt in a 12.00 inch pattern. A mitt such as the 2017 Rawlings Gamer Youth Pro Taper 12" First Base Mitt: GFM16B would thus be an ideal fit. Whether to use a 12.00 inch pattern or a 12.50 like the Rawlings Sandlot Series 12.5" First Base Mitt: SFM18 will come down to personal preference as far as what size feels comfortable to the player. With that being said, the Rawlings Sandlot Series 12.5" First Base Mitt: SFM18 will have more of an adult fit with relationship to the wrist opening. This may benefit a bigger 11 year old player compared to the youth pro taper fit of the Rawlings Gamer.
At your player's age, it would be hard to suggest a better first base mitt than the 2017 Rawlings Gamer Youth Pro Taper 12" First Base Mitt: GFM16B. It features a snug fit for a young player, and the overall quality of the glove would be hard to beat in another youth model.