RE: Fuel? Glow plugs?
James, if I understood you correctly, your using Byrons' 15% Nitro 18% oil ie aviation fuel? If so and setting any tuning errors aside, that could account for the bog and/or lackluster performance.
Do not substitute Airplane fuel for car fuel or vise versa. Car engines typically require a higher percentage of nitro (20% minimum) with substantually less oil content, usually around 12% (max). For your sons 2.5 application, I would suggest Byrons 20% Race Blend w/ a McCoy #8 glow plug. Note: Only use OS plugs in OS engines..