Transitions usually get a forward going horse!
It is there natural attitude when it comes to riding
You can’t not make your horse lazy
I don’t think your problem here is that your horse is lazy, I think he has just learned that he can ignore your legs and other tools.
Teach your horse with groundwork and liberty work how to respond to light signals and vioce commands. Than riding will be much more pleasant for both of you.
Hope that helps...
The issue here is not that your horse is lazy, rather your horse has learned that he can ignore the leg. You can kick him on as much as you like; if he's learned he doesn't have to move, he won't.
Re-training is required - not spurs or crops.
I'd ensure that you gain your horses respect before getting on him through using a round pen/lunge pen. The ONLY way to get a horse to respect you is to move his feet. He has to understand that if you ask him to move, he moves, otherwise, like a herd leader would do, he would get a nip (or whip, in our case) to the hind.
I highly recommend watching Clinton Anderson's videos on gaining a horses respect. Unless your horse respects you, he has no reason to move when you ask him to.
After doing the essential groundwork, only then hop on his back. Do NOT kick! Squeezing is the best option, since you can start off asking him to go forward lightly, then increase the pressure on his sides if he does not move off. Kicking takes away the pressure too quickly and your horse learns to stand, since no pressure = right answer and pressure = wrong answer. (if you need any of this clarifying please message me)
So, begin by squeezing his sides lightly but clearly. If he does not move off your leg, squeeze him more and do not release that squeeze. Only once he moves you can stop squeezing his sides with your leg. This way, he will learn that if you squeeze, he must walk on to get rid of the pressure on his sides. Make the wrong thing hard and the right thing easy!
If he still fails to move forward after squeezing as much as you can, ask someone on the ground to smack the ground with a lunge whip to increase the pressure (you continue squeezing to let him know that not moving forward is the wrong answer). If he still fails to move, ask them to whip his hind, since this should send him forward. Keep a loose rein since you don't want him to catch his mouth if he jerks forward.
Repeat above until he responds to the lightest squeeze. Take baby steps and do not rush him. Start getting him to respond correctly in walk first, then after that is perfected move onto trot and so on. Remember, gain his respect on the ground first before returning to his back.
Try focusing on the absence of leg, so your horse reacts to it when needed and doesn't become non responsive, if you're always using it they learn to ignore it
Also transitions are a great exercise to getting your horse to listen to your leg better.
Kicking is also not an appropriate method, you want to squeeze, possibly use a dressage whip so you can use it as a quick reminder if you get no response.
For a schooling purpose, everytime you squeeze for an upwards transition give a small tap, as a reminder that your leg on means forward!
Goodluck, lazy horses can be frustrating
Keep him (or her) on his toes. Spurs will help but be sure to use them correctly. Vitimans are also a good idea. In your ride keep it interesting with transitions, etc