@Ricardo Chapiano You can easily shape his sitting or holding his foot to have the tips either filed or clipped off. Shaping is breaking the target behaviour down into tiny manageable steps or approximations. This is how I taught Ollie to sit still for me as I clip his. With Kobe, I make a game of it and clip one here and there.
Think about what you want the final behaviour to look like. It could be sitting on the perch whilst you clip the nails from underneath (that's what I do with Ollie) or it could be him holding onto the sides of the cage whilst you clip one or two that way. For the latter you would have to shape him to hold the bars first if he doesn't do that already.
Then think about what you are going to use for reinforcement. A favourite treat works well. To be different the other day I used a silly noise to reinforce the steps for Ollie.
First step might be you bringing the clippers/file as close as to provoke no reaction. Treat!
Next step, bird sits with relaxed body language (RBL) as you bring the clippers an inch closer. Treat!
Step 3 - bird sits showing RBL as you bring the clippers slightly closer. Treat!
Step 4 - bird sits showing RBL as you bring the clippers closer still. Treat!
Step 5 - bird sits showing RBL as you bring the clippers even closer. Treat!
You may want to call it a day there depending on the bird. If the bird fails a step (RBL is not so relaxed!) then back up and repeat the last successful step. Then move forwards again but in even smaller approximations.
Step 6 - bird sits showing RBL as you bring the clippers close. Treat!
Step 7 - bird sits showing RBL as you touch a nail with the clippers. Big treat!
Step 8 - bird sits showing RBL as you touch another nail with the clippers. Treat!
Step 9 - bird sits showing RBL as you open the clippers next to the nail. Treat!
Step 10 - bird sits showing RBL as you touch the tip of the nail with open clippers. Treat!
Step 11 - bird sits showing RBL as you clip the end of the nail. BIG TREAT!
Watch the bird always - he will tell you how fast or how slow he wants to go - you might speed through some of the approximations and have to break others down even smaller. The above might be achieved over a few sessions in a day or over a few sessions over a few days... or longer. Always end on a successful step or approximation BEFORE the bird gets bored or uncomfortable.
Every time you want to clip his nails you will have to shape the process from the beginning. But as he gets used to it you will be speeding through the steps and probably missing a whole lot out. You will also have to shape the process with each toe since each is different, ie. just because you can cut the tip off a front nail, doesn't mean you can clip a back one just like that. You'll have to start from the beginning again.
It works - I did this with Ollie who was not tame when he first arrived. Like you with yours, I really didn't want to stress him out by taking him to the vet and lose the little trust and progress we had made. Sometimes I use skritches (he adores being skritched on the head) as the reinforcer and as I said above, I tried and succeeded using a silly noise that I had paired with the head skritch. As you can see from the pic below, I am not trying to hide the clippers - he is in control of what is happening. Holding his nail like that as the clippers first touch and then clip, earns reinforcement be it a treat, head scritch or silly noise. If whatever I was using wasn't reinforcing enough, be assured he would not stay there: