You've asked a really hard question, what to say to him, and only you know him the best. We can piece things together based on your replies.
You say he's desperate for sex & not getting it is making him crazy. So does he just want the release of sex or does he miss the relationship between the 2 of you?

You say that he lashes out & says the relationship is over, then comes back & says he doesn't want it to be. So he explodes in anger, for not getting his way? then feels remorse? and says he just wants you. So he's placing sexual pressure on you. Coercing your consent for sex. This is creating a difficult power struggle that can lead to other difficulties in making joint decisions.

You say he wants sex to go back to normal....what's normal? AFA I know sex in relationships have an ebb & flow to them. Lust & passion fade, but is replaced by other emotions.

You say he's not spontaneous anymore. Does that mean you want him to take charge in the bedroom? Be more "dominant?" If he's less romantic, can you initiate sex when you want it? What type of power situation is going on in the bedroom?

You asked what should you say. That's hard & ive learned that instead of leading with statements, sometimes it's better to lead with questions. Open ended questions so this put the ball in his court to answer & give you more information so you can make better decisions. Keep asking questions until you're confident that you see his point of view.

Also what other life factors play into this? Things like work other pressures, relationships, stress, money, society issues etc. what else can you see in hindsight that lead to this?

I'm not sure this has helped at all, but I do wish you the best. And yes I feel a personality can change due to sex. My SO did as soon as I started to say no in the bedroom & stopped being at the beckon call. I'm trying to regain my own power.