I'm not saying that he's not at work, by any means, but I've worked in finance for years. My job meant sometimes I really did have to be there late (think crisis management). Most of the senior men who stayed that late were choosing to do so. Yes, they probably got spreadsheets done, or meetings, and it was the best time to be around if you needed to chat to someone about something or get some advice - but they could have gone, if they wanted to.
So from my experience, in the Summer he has typically chosen to leave earlier and come home to you, and this Summer he has unilaterally decided to go to the gym (for a pretty long time?!) after work instead. I don't at all begrudge people the gym, I need it to destress if I'm working long hours, but I'd be querying both why he didn't discuss this with you and what has changed.
From your last comment, it could just be that he doesn't know how to settle his son if he's grizzly/upset and feels a bit useless. Or he could be opting out of family life, or I guess there could be someone else. You're in the best position to know which is more likely.
I'm with @coffeeonthesofa though, he could spend that time in a home office - which would be better or worse depending on whether having him in the house but not helping would be more or less frustrating for you and DS!