I would not ASK for an x-ray or an Ultrasound. I would ask what he thought you needed to do. You will establish a better relationship with him asking questions than telling him what to do. I would hope he suggests x-rays and can take good ones of the splint bone area, first. We take problem horses to the Vet Clinic because they have better x-ray machines there and they can develop them to see if they need another view. Most mobile units are not as good and require another Vet call after they have been looked at.
I would not opt for an Ultrasound or an MRI of anything without knowing that you are spending the extra diagnostic money on what is actually causing the lameness. I have seen people spend $2500.00 on MRIs and Ultrasounds of the wrong area when the lameness was found by a different Vet and was in an entirely different place. Experience has taught me to get a specific area (usually a joint or a specific ligament or tendon) diagnosed as the problem area before using more expensive techniques. Then, I ask the Vet if the further use of diagnostic tools will actually help him decide exactly what he will do and what those different options will actually help with the recovery or healing.
If turnout or joint injections are the reasonable options for a particular horse instead of a $10,000.00 surgery, then I do not spend thousands of $$$ to get details that are really only useful to a surgeon. If the diagnostic tools recommended will help determine the course of action and the outcome, then, I go ahead with them. These are things you need to discuss with the Vet BEFORE he comes out. You need to ask him if he can take better x-rays at his clinic if they are necessary and needs to be able to look at them before he does more diagnostic work. You are definitely waaaay ahead with a clinic visit and not a farm call if x-rays are indicated.