What does my lemon tree need? Help!

I planted a few seeds of a lemon from a friend's organic tree July of 2016. The tree has been growing great. It smells beautiful and is developing great little thorns. My issue is the a couple of the leaves are tightly curled under and some are yellowed. I have not yet used any fertilizer as I am not to familiar with what type I should use as well as if the tree is old enough, if that even makes a difference. It has spent most of its life in a good majority of sun, but most recently has been in an area with indirect sunlight throughout the day in fear of the possibility it was getting too much sun. Also not sure if that's a thing. I have it in a basic cactus/citrus soil mix and let it just about dry out inbetween watering. It seems like a pretty healthy tree, but I know it's trying to tell me something. Not sure what type of lemon tree it is specifically, but the lemon the seed came from was off a rather large fruit bearing tree and the fruit quite bitter and juicy. If anyone has any ideas what this little guy may need I would appreciate any tips and recommendations! Pictures below

I'd guess an iron deficiency possibly due to not sufficiently acidic potting medium. I'd try a for a foliar feed.
I'm looking forward to other's opinions though...

Sounds like someone at least started you with good advice. Something to remember with all citrus is that they are very hungry plants. They're one of the few plants who can handle straight urine without burning the roots. If you're not squeamish, that is actually a very sound practice for adding nitrogen and nearly (if not completely) all the micro nutrients the plant needs. If you're uncomfortable with such an organic solution, choose an organic fertilizer with a high amount of nitrogen (the first number).

I agree with Casie.  Citrus are nitrogen pigs.  They like a good shot of N regularly, and nothing does that more effectively than urine.

However, with all the N they'll put out a flush of new growth and that makes them a target for leaf miners, aphids and other pests.  So finding that sweet spot (enough, but not too much) is important.

They don't like to be sprayed with water, so if you've got a sprinkler that is hitting it directly, move it.  Don't give it too much water -- don't water it daily.  Once its established (after a year or two), water it deeply once or twice a week (if you've got good draining soil) and then let it dry out between waterings.

Lemons are tougher than most citrus, (with the exception of Meyer lemons, which can be finicky until established).  Once your lemon is established, they tend to be prolific and very hearty.  I almost completely ignore my tree now, with the exception of occasionally taking a pee around it, and hacking off the long 8 to 10 foot branches that shoot straight up every spring.  Once established, you get 500 lemons a year --- more than you can ever use.

Looking at your pictures again --- your tree looks very healthy.  Nothing there that would concern me.  Yes, bump up the N, but it doesn't look bad.

They like the full sun, but you may want to shield the pot from taking a lot of sun.  A black plastic pot is a heat-sink, and the roots will get really hot if the sun is directly hitting the side of the pot.  A tree in the ground doesn't experience that kind of temperature fluctuation --- maybe 5 to 10 degrees max over the course of a day.  Whereas a potted plant may see the temperature fluctuate 30 or 40 degrees if the afternoon sun is baking the side of the pot.  Can you put a larger light-colored pot around it, or create some sort of insulated shade around the pot so it isn't getting hit by direct sunlight.  The old adage is "head in the sun, toes in the shade".  That will also reduce the need to water so frequently, so your roots will stay more evenly moist without the spike of too wet, too dry.

Are you planning on putting that plant in the ground soon?  This is a good time of the year to do so.