His favorite champions are Teemo (I know, I know) Ziggs and Caitlyn. He wants someone more along the lines of a solo lane, though. So anyone like Teemo or Ziggs or something cool that not a lot of people play.
I've been trying to think of some easier champs that are fun and not played often. I recommended Kennen but he was unsure if he'd like him.
Renkleton. Be a man.
Kennen would be a good one. Lux comes to mind, if you wanna go a little harder n the difficulty scale you have brand or liss (played a fair amount haha). 
Well i'd say velkoz he's a consistently strong and underestimated midlaner with lots of poke and really isnt played often
Gangplank is an interesting solo laner that not a lot of people play regularly. Jungle is generally solo and Elyse doesn't see the use she once did. you can solo with Garen - I like him a lot. Heimer or Vel - though they may be more popular. you could lane with like any champ, you just have to know them and your opponent pretty well. I take Skarner top sometimes. No one does that...
If you're looking for straight up fun in the mid lane. 
Straight up fun in the top lane. 
Jungle 
Good luck and have fun!
But i think it more or less dpends on your father in law.. What is he like? does he like skill shot or point clicks?
By what your saying it sems he like point clicks and not like skill shots you have to weave threw minions.. If he like cait and teemo he seems to like traps.. or champions that can place things..
So what does he like from those 3 champsions..
trap laying champions that can solo lane Nidilee (Bear traps) Maokai (Sapplings!) Teemo - (Already likes) Heimer - (TRRETS) Ziggs - (already likes)
*Point and clicks he might like.. * Rumble (Half mele half ranged) Gnar - (Sometimes mele but mostly ranged) Yorick (Be surprised how much his things hurt and can attack you from so far) Malphite (Hits like a rock)
Caitlyn is a point and click adc with a longranged ult.. Ziggs has a longranged ult too... teemo's ult is all over the map! perhaps he could try these adcs.. Jinx kog maw (Ive been sniped long ranged by him soooooo many times) Ezreal.
Very long ranged ults good for securing kills when they try to get away.
You can like all kinds of champions but it can depend what speaks to you.. what are his interests?
