The sorcerer spell list is significantly more damage-focused than the druid list, which tends towards debuffs and utility spells. That doesn't make it inherently unbalanced as such, but it might be of concern. I might suggest using the druid list, and if there are a couple of specific sorcerer cantrips or spells that seem particularly apt for the character, add them to his list, rather than just switching out the entire spell list. It only takes one or two cantrips to give the character a unique flavor, without going to the extreme of rewriting the whole class list.
Switching Wild Shape into, let's call it "Monster Shape", is a tough question. Honestly, it really does depend on which subclass he takes, because a Moon Druid's expanded CR choices will make all the difference.
A non-moon subclass may find the monster choices very limiting, because there just aren't that many weak monstrosities that don't have a fly speed. I think you'll find that at 2nd level (CR 1/4) he'll have no valid choices, and only a handful at 4th level (CR 1/2). This may, of course, change with future book releases. Once he hits 8th level there are maybe a few interesting Monster Shape forms, but still very limited, and weak for that level to boot. A normal non-moon druid at that level tends to use wildshape for utility effects rather than to get cool combat forms, and some of that utility will be lost just because a Darkmantle or whatever is a lot more notable than a raven, rat, or even bear.
Using Circle of the Moon adds quite a few forms. It's still very limited, but maybe not so bad; the big issue there is as you level up, you'll be adding monsters with bizarre abilities to the list, some of which are fine for their CR in the context of a single fight but may be gamebreaking if a PC can just use them any old time he wants. Most of the really bad stuff is forever out of reach, but at higher levels you will see stuff like Lamias, Gorgons, and Medusae coming into play. And Mimics are in at 6th level, which all by itself might give you pause.
So what can you do? Well, I feel people often try to over-complicate things by rewriting huge swathes of rules text to fit with a weird idea, rather than taking the safer and easier path of reskinning the existing rules with new descriptions to match the flavor of what they want to do, and possibly making one or two little tweaks when the mechanics don't quite work as written.
My suggestion in this case is to use Wild Shape as it's written, but change the flavor by reskinning each of the Beast forms to describe some quasi-human monstrosity instead. A dire wolf becomes a hulking wolf-man. A giant eagle becomes a harpy or even griffon form. A warhorse form is described as a centaur or minotaur, a brown bear is an owlbear or half-troll, a giant bat is the Man-Bat from DC Comics. Whatever. I think that sort of thing could easily give you the flavor of turning into bizarre monsters without getting into the mechanical difficulties of the actual Monstrosity creature type. (You'll still have the roleplay effects of being a gross monster rather than just a regular horse or what-have-you, so this might work best if he goes for Moon druid.)
The only mechanical changes that might come up are whether some monster forms can talk or use their hands, but even then it's less common than it might look. A giant wolf-man that's secretly a direwolf probably can't speak like a human, just growls and howls, and his massive clawed hands could easily be useless for anything but the most basic grasping that a wolf's muzzle could accomplish just as easily. You could have his forms use the "monstrosity" type rather than "beast" -- the only real game effect there is immunity to a tiny list of spells and effects. ("Animal Friendship" and "Dominate Beast" are the only ones that really come to mind, off the top of my head, and those can be largely duplicated by higher level spells -- they're just easier to get to if you're targeting a beast.)