Hi, 35 year old man here and this all sounds VERY familiar.
Apologies in advance for the long post/life story.
I lost a lot of my focus and determination (in all aspects of life) over the past 2 years. My wife and I have been married for 3 years (together for 8-ish) and I still love her very much but find myself stuck in a rut.
I have a good job but can't really be bothered to apply myself to it. I let a business I was running on the side fail because of a lack of motivation. I don't eat properly (normally once a day), have increased my alcohol consumption significantly and don't really find pleasure in anything.
I ALSO tried the gym and kept it up for about a week.
Firstly, I think it's best for him to seek proper help as it may be depression (I have a history of it). My advice is purely from my own experience and please take it as that and only that.
I THINK I've managed to get the bottom of it and, as trivial term as it is, I think that 'mid life crisis' covers it.
I started going to the pub after work (virtually every night). I work alone and this was my way of socialising a bit and unwinding before going home. My wife always knew where I was and, because I was coming home in a better mood, it was beneficial for both of us.
Obviously going to the pub every night isn't very good for you so I started walking. Nothing specific, just around local estates etc for an hour or so (and increasingly longer). This actually gave me the headspace (away from home, work and other people) to get, I believe, to the bottom of my problem.
One night it was really cold, blowing a gale and hammering it down...and I loved it! It soon occurred to me WHY I loved it.
I left school/college at 18 having been badly bullied for 3-4 years previous. At that time, I moved away and got a job at Butlin's doing security work. This was mainly working on the doors but after the clubs shut at 2-3am we used to do mobile patrols for a few hours, checking for stragglers/anti-social behaviour/noise etc. This was a very solitary job and, being on the coast, during the colder months it was very cold, very windy and often hammering down.
I realised that THIS was why I was getting so much from walking because, being outside alone, I was that 18 year old version of myself again. I had got older, my life had changed dramatically but it turns out that, in my head, I am exactly the same person.
I found myself comparing the thoughts of 18 year old me and what I had ACTUALLY done with my life...and I cried. Don't get me wrong, I have a good job, a lovely wife and a stepdaughter but I realise that I'd buried that free, ambitious version of myself under a sea of responsibility and work.
I'm not speaking for all men but I KNOW for a fact that I never wanted to grow up, I never planned to have children (the reason I'm on MN is because I didn't know where to start when I became a stepdad and wanted to do a good job of it...apparently I do) and I genuinely have always been happy in my own company.
Oh...and yes, whilst I didn't actively seek an affair, I DID find myself messaging girls from my past and was shocked to find that I still got a 'buzz' when I heard back from them. It did genuinely feel like I was 18 again, very, very strange. So, whilst I didn't have an affair (and don't intend to), I guess we use affairs as a way to try and reconnect with our care-free youth (bloody stupid and destructive way of doing it mind you!).
It's something I'm still working through and can't say that I'm 100% happy but this certainly helped me identify why I might feel the way I do. I'm coming to accept (although I doubt I ever will fully) that I can never go back and any attempt to would involve losing the most important person in my life (as well as my home, job and all the material shit that seems so important but isn't).
I didn't want to grow up but it turns out that I did and I need to change the way I think about that.
Like I say, I'm still struggling, but I feel that by allowing myself time and space to retreat into my own head and get to grips with myself, I have managed to 'survive' my midlife crisis without damaging my marriage (I did buy a convertible though...how cliche!).
Sorry to ramble but your story sounded so familiar. I'd certainly recommend professional help but mild exercise, space to think and being brutally honest with myself certainly helped me.
Sending you my best as I know I'm not exactly a joy to live with all the time.
P