It is an interesting n=2 experiment.
While there are many other possibilities, since we do not know you day to day and there might be something that did not occur to you (happens to me all the time!), it sounds like one of three possibilities (to restate the obvious!) Do not mean to make assumptions or presumptions just wondering on paper
The fake sugar either kicked you out of ketosis resulting in some calories being fully burned instead of partially burned, too efficiently, so they ended up being stored, changed your gut microbiome in an unfavorable way or it sparked an insulin response that caused you to hold on to fat instead of burning it. Did you drink it often throughout the day, possibly by itself? Personally I will drink the occasional Bai and it does not seem to cause a problem but that may not be everyone. Giving it up is good.
The lack of activity reduced your calorie expenditure, which while unlikely to cause a huge weight gain, since you do not burn off that much in exercise, the lack of general activity (ie you do not go out much in your daily life, running errands, driving or taking public transport to work the way you would have pre-Covid) caused a weight gain or the most likely of these possibilities, you have lost muscle from not working out which burns more calories than fat. I feel like this is something that has happened to me. Packages are harder to carry, and I really spend very little time outside my house.
You and your son may have undergone some other change in your life either as a result of the pandemic or something unrelated. Either way it has triggered additional stress which causes weight gain.Exercise is a good stress reliever for some people so without it that may be an issue. Throwing this last one out there as something to consider
In terms of narrowing it down, if your son is willing, you could measure his blood glucose about an a half hour after waking up, have him drink a Gatorade, and see if his glucose level changes, 30 minutes and an hour later. Note a large drop means an insulin response. You could do the same thing but that means returning to drinking the poison which may trigger something for you.
You could also work on doing some muscle building exercises at home, working out with weights a few minutes every other day. It will not burn much in calories, but it may build enough muscle to reduce your weight. Also if you really have not been doing much while home, perhaps consider some general movement to replace not commuting and things like that. Possibly some meditation or other stress relief if that is an issue.
Have you ever tracked what you eat? You may want to do it for a little while to see if the poster who said you may be eating too many calories is correct also to see if you are eating more than you did a year ago.
Finally consider eating foods or taking supplements to help your microbiome