Hi,
I would probably invest a little dough in a cooking scale to get a better idea of his weight and the weight of his food. You don't need devastatingly accurate but you need to be able to trust it's reasonably close.
If he is two years old and 275g then you really might want to think about bumping up his food to 10-15% of bodyweight every 5 days and see how he gets on with that.
I like to grow my snakes fairly slowly but my black pastel is 365g at around 9-10 months old. I fed him once every five days for the first 6 months (roughly ), starting with one half grown mouse, then graduating to two half grown mice and finally changing to interval to every 7 days when he was ready for full grown mice. I have kept him at one full grown mouse every 7 days since but he is almost ready for two full grown mice at a time - whether he takes them of course is up to him.
What are the hot and cold end temps as measured in his hides? 75-80f isn't a terrible ambient ( though 80-82f would be best ) but it is serious if the snake cannot get access to higher temps.
dr del