Regression at the age of 4+

Are you absolutely sure he hasn’t had a bad experience? My daughter wasn’t performing very well in her swimming lessons and we noticed one week when some of the other children were late that her demeanour completely shifted when she saw them coming. They are very young and their behaviour was very unpredictable and they would often end up splashing her when she has sensory problems with getting a wet face. She does much better in a smaller class with slightly older children. Swimming pools are also very loud humid places which smell strongly of chlorine everywhere, it’s possible something more subtle has happened to put him off

Also some children develop irrational fears around the swimming pool mechanics. Fear of being sucked into event. Fear of a shark or something else nasty swimming out of a vent. It depends how rational your child is.

I really started to hate swimming when the local council significantly increased the amount of chlorine in the water. It was very much a sensory thing. Also bullying between boys in the men’s changing room is quite common.

Many times what is happening with and autistic child isn't regression at all, but a shift in focus.

A new discipline can be all-demanding for us mentally, which might mean any particular thing we externally appeared to be fine with, might need to be put on hold for some time until we've mastered a new skill and then the process of integration of skills can even be its own task. The way we learn best is through a full immersion which will mean other things need to be put on hold in order to learn properly. But also, as I recall my grandmother used to reinforce we do things one-thing-at-a-time. And as an adult, I now find that if I block out and appropriate very focused time to a thing, I am more efficient and a thing is done properly. Not everyone needs to operate this way. But when young, I didn't know that multitasking was actually making me accident prone, less productive and prohibiting learning. In fact, most of us would probably be much further in life if we only had 2-3 subjects in school unbroken for blocks of time.

But, I'm assuming he's not afraid of the bath as well. Another thought is problems a pool might present, which previously he may not have noticed. When one is young, we can be overwhelmed and sometimes just lost in our head in one thing. As we grow, we grow in awareness. It could also be that a sensory element may have changed. While I'm much older, I recall when all these new lights were suddenly put into train stations and places like M&S, now they're everywhere - blinding and aggressive. Places I used to tolerate, I now feel assaulted in. And unless you have shaded goggles, sunglasses are of little use, the lights are laser beams hitting the top of the eyes in very harsh ways.

These are some nuances that sometimes we can't always articulate when young. But perhaps if he's in a learning phase with something else, he might come back to swimming in the future as if nothing had changed.

I'm not sure that stopping liking to do something, or even fearing it, is really a regression in a developmental sense. It is usually things like loosing the ability to speak in phrases and sentences, or the loss of other communication skills. Apparently, regression can, however, occur as late as 7 years, see: https://www.spectrumnews.org/news/regression-marks-one-five-autism-cases-large-study-finds/

My wife was always fearful of being in water anywhere near her depth, but learned to swim reasonably competently while she was pregnant with our first child. However, when we went on our first holiday after the birth, when our child wad about 5 months old, my wife's confidence in the water had evaporated completely, not through any traumatic experience, and ever since she had her old fear of water in its original form. I think that these things can happen to anyone of any age.