I do think its quite common for them to start to get a bit picky around this age, - Humphrey did and I‘ve seen several similar posts on here.
I would start by not giving any treats at all, just to see if he eats better when he‘s really hungry. You could also take his bowl away after 15 minutes and not offer it to him again until the next mealtime. Obviously you don‘t want to make him miss too many meals but it won‘t hurt him to go hungry for a day and it will give you an idea whether its the food or something else. Be careful not to make a show of being too worried, (the little wotsits pick up on these things so quickly and use them to their advantage!
) Try not to get into the habit of leaving the bowl down, - I did that with my first dog and it became a real pain, it took him all day to finish his food which wasn‘t good in the hot weather. Could his teeth be bothering him?
It could simply be that he has infact gone off that brand of food, a spoonful of wet mixed in might also help, - (my last dog prefered a mixture of both), and if all else fails you might be best trying another one but I‘d do that as a last resort.
Hope that helps ?