Nothing fits, what do I do?

I'm having a hard time finding my son pants that fit. 😩 12 month pants are too small, as they are tight and he looks like he's prepping for high waters. But 18 month do not fit him around the waist. I haven't been able to find pants with the things on the inside to tighten them. As it stands right now I have him in 18mo pants that are tied with string.. 
What do I do? Or where can I go to find pants that are able to tighten up , so I'm not worried about the strings ?