I think you’ve got a good plan in place. It’s always a good idea to make sure there isn’t any physical that is giving him reluctance to move forward.
But with that said, I have NEVER had a horse not move forward because of pain. Had lots of others things and surely have had my fair share of lameness over the years, but forward means forward means forward. Period.
Now, in general, those “more whoa than go” horses can be absolute blessing for new riders, timid, etc because you know they are SAFE and they will not run off.
One of my retirees is 19 this year and he’s the type of horse that my kids (ages 7 and 9) can barely get into a trot with much convincing. Whereas I can hop on him and do an immediate walk to lope transition. He knows who is on his back and what his job is. When my kids get to be better, and more confident riders, they’ll get that out of him too. Until then, I am content with him liking to be lazy and not put forth much effort.
Now…horse like yours, well, yes, that gets rather annoying when you are the experienced rider and you feel like it’s a workout to keep him moving.
So I would tell you it’s time to be stern with him. Seriously. Go means go, and it means go now. (not later)
Have a whip ready. Start off on a standstill and ask very nicely for him to move into a walk with a nice squeeze from your calves. Likely, he won’t move (based on what you are telling us). Then you SMACK HIM and you mean it. He had better lurch forward. Let him go a few strides, then pull him up, stop and settle. Than ask again. From a standstill, ask very nicely to move into a walk with a squeeze from your calves. If he does not immediately and briskly give you that stop-to-walk transition, then you hit him again. You repeat until he gives you a nice transition into the walk. Make sure to never “end” your lesson whipping him. If you do this right, you shouldn’t have to repeat more than 2 or 3 times.
He is 9 years old. He knows better. He knows what you are asking. He’s just putting in no effort to do it. You have to TRAIN the effort. This might be something where you need to ride him only yourself (and not your husband) for a while to work on TRAINING that. It will be something you are going to have to reinforce and BE CONSISTENT with because if you put your less than experienced husband on him, he is going to get away with it because your husband won’t be able to do the same things you do. But you can still work on it.
You can do this same method for everything else. Maybe that’s maintaining the trot you asked for. You should not need to “beg” him to stay in a trot, nor NAG on him to stay in a trot, for example. He should maintain the pace you asked for until you direct him otherwise.
I would tell you to have some confidence in yourself. EXPECT HIM to do what you ask and to do it BRISKLY. Be as firm as necessary but do not be afraid to be firm. You aren’t beating on him, but you are setting very, very clear rules so that he understands what is going to happen if he chooses to be lazy. Don’t stop him from making the choice but be ready to show him when it’s the wrong choice.
And again, he’s been trained to be lazy because he has been allowed to be lazy.
You should never have to beg to keep him moving forward. That’s his job. He should just do it.
If he stops and plants his feet, then you do whatever it is that you have to do to get his feet to move. It might look really ugly but those feet need to move somewhere. Trust your judgement on if you also have to (for example) turn him to the side, etc, to get him moving, but the feet must move!
And another thing I’ll add with the whipping method above, when he starts moving and does it nicely and you don’t have to use the whip, LEAVE HIM BE!!! Do not allow yourself to fall back into the habit of nagging on him and begging him to keep moving forward. So keep your legs off him and let him do his job of moving forward, like you asked.
If all you do is nag him to move forward, pretty soon (as you have it) your legs mean nothing to him because he is already moving forward yet your bumping on him to keep moving forward. So why would he move forward?
Be very, very very clear with your legs.