SandyMcD Great question. When talking to him about changing his clothes, try to use the simplest language possible (without explanation). You can bring his shirt for example and say, "can change your shirt?". More of an explanation can become more confusing for him. You can even bring two options, and say "pick a shirt to put on". If he has a favorite shirt or a favorite pair of pants you can offer those.
Sometime it's helpful if you use a little white lie. Depending on his cognitive abilities, a simple explanation could be helpful. You can say "you need to change because ___ is coming over".
Something else to consider - if his clothes are only slightly dirty, and with something harmless (like juice or food), it may be easier to not push it. Unless it's soiled with urine/feces, which then can become a health issue. But if it's harmless to not push it until he wants to, that's okay too.
I hope this helps a bit.
