I need to get home to see what the spellcasting part of a 5 Paladin/15 cleric would be. I think that is going to be what i suggest.
I think he likes to melee and probably at first would rather have been a fighter but the group needed a healer so he tried cleric but tried to get his fighter (High) while playing a cleric. He knows he isn't ever going to be as good a melee guy as the fighter....Fighters get tons more attacks and specials BUT he seems to be happy being a melee oriented cleric. He would happily give up greater spell damage for increased melee damage.
Why does he want the extra attack so much? I think his Pathfinder and 3.5 clerics got two attacks and he just considers that the baseline for melee class's. Like I said I could tell him right now..you get a extra melee attack at 5th but lose all attack spells and he would be thrilled.
Come to think of it i only remember him casting ranged attack spells in pathfinder once.
So it isn't a power grab for more damage or anything like that. It just fits his mental image of what a Battle Cleric should be.
You know I think I just talked myself into giving him a extra attack at 6th level. House Rule #23 coming up. No reason to have him go Paladin and all that hassle for a tiny bit more combat ability...I'll just take his war feature for it and call it a day and see how it ends up.
I would start analyzing this from another angle, but since I would probably arrive at what you just decided yourself, I'll be brief
What is his Strength and Wisdom scores?
Because unless he's gunning for an 18-20 Strength, there will be absolutely no problems with him doing two attacks.
Especially if he's a board-and-sword fighter (since only ranged, greatweapon and polearm fighters can really wreck the balance, and then only if feats are enabled).
And even if he is going for Strength 20, his decreased spell save DC will more than adequately balance things around.
I'm getting that he wants to be able to do the basic stuff of a fighter: attack twice and wear heavy armor, and still heal like a Cleric.
And you know what? That's far from wrecking the game.
He will still be able to provide the sheer utility of a full spellcaster. Things like high level divinations, abjurations and teleportations. And that stuff will benefit the whole group, especially if nobody else is playing a singleclassed wizard.