The dynamics of relationships change with time as each partner becomes more familiar with the other. The "in love" stages changes and it may be one of comfort as well as complacency.
He does not have to necessarily be cheating. It may be that he had become too relaxed in this relationship and thinks that you'd be there for him regardless of what his behavior is like.
If you do not feel a spark and emotional connection with him, then try to introduce some spontaneity in the relationship. Continue to try to do the things that the two of you enjoy. Do not let him feel that you're chasing after him. If he does not respond to your messages right away, just let him know that he has your contact info and let him go after you.
Yes, a long distance relationship has its own disadvantages (physical intimacy, doing things together, etc) But, it can be a strong relationship as long as both partners are committed and open to make compromises when needed.
If the distance is not too long, you could plan a surprise visit or at least the two of you can meet some place halfway and spend the weekend together at a B&B.
There may not be a lot of signs of cheating for you to detect if he resides away (such as physical evidence) unless he indicates something to make you believe that something is happening. The same applies for him and what you're up to. If he seems preoccupied and less affectionate, that is something to look into (it does not always mean he's cheating but any withdrawal like that should be explored)
The two of you should try to find time when you can speak in person about the goals of the relationship and each other's expectations.