OP, this is not uncommon. We've been doing this for 4 years now and the kids spirit goes up and down naturally. You can't train these kids at a high tempo constantly, they won't be excited and have that spark all the time. In fact, playing so much of 1 sport at age 8 becomes tough and it is kinda like having a Job now...your kid is going through the normal growth cycle of figuring out there is more to all these practices than doing the same things he used to like to do when he first started. Yes, it is hard for the kids to always understand what the coach wants. My DD, at age 8, had a 55 year old man who was a former Women's D1 Head Coach....he had complex drills and the learning curve was steep. It is 100% ok for your kid to skip a practice at his age. It is possible he is dealing with not being able to shine as much as he once did and he has to watch his friends shine now.
-It's normal. This dedication to 1 youth sport is a long journey.
-Also, it's ok to check in with the Coach if this continues for more than a few weeks.
-Kids are fickle and will be for years. But, please remember, it is a massive benefit of team sports, learning how to get along with others and a the coach -this will serve your child well later in life!