Really could be a variety of reasons. -could’ve gotten in trouble for being on his phone too much -is comfortable in the relationship now and doesn’t feel the need to put in that much effort into messaging -feels like your smothering him, maybe someone at work made a joke about how often he’s messaging you. -he’s cheating -etc.
Definitely ask him about it first, if he tells you what the reason is-awesome! You can adjust. If he brushes it off and says idk why you’re talking about. go with the smothering answer and stop messaging him. It will help with all the other possibilities. He won’t get in trouble at work, he will start to miss you-which will make him put a little more effort and reach out eventually. you won’t be smothering him if that’s the reason. If he’s cheating, nothing helps.