ESY can be tricky...all depends on the district, some districts will automatically say yes and some will fight you till the end.
OK...so 1. myth they will try to throw at you is about regression. Regression is often the reason that ESY will be included in an IEP but it is not a requirement in order to get it. "catching" your child up to where they should be is a perfectly legitimate reason for them to provide ESY. To that end there can be no "single criteria" for determining whether or not ESY is needed.
An independent assessment is expensive, and unless you are sitting on a pot of gold don't pay for one simply to procure ESY services. You'd be better off putting that money into getting services for him in the summer on your own.
If there are no regression/recoupment issues and he's still 2 years behind, I'm of the opinion that his IEP could use some tweaking. Are the reading services, goals and benchmarks aggressive enough to help him get caught up? Not sure if it is. I have asked for more reading hours and to change it to am since he has a hard time in the pm but have been told that it is what it is. It sounds from your post like he is capable of it? I believe he is it just takes so much more for him to learn. I'm wondering if your energy would be better spent fighting for more/better reading services throughout the school year?
I don't know your area well, but (and this is rare for me to say) sometimes when they say there are "no programs" to fit his needs this is actually true (in the summer months). Be careful that you know what you are asking for. You don't want him to just end up wharehoused in a regular summer school just to appease you. That's not going to help, and in some cases can even hurt. Maybe what you need are creative ESY services, one of the things courts will look at is whether or not the parents can provide a structured educational environment during the summer. Maybe a district laptop loaded with a good interactive reading program that you can use with him during the summer would be more beneficial?
I'm a little worried about this as well.
The other thing is remember not to wait too long to request ESY, most districts will try to "schedule" this discussion during the last IEP meeting of the year. The courts frown on districts who employ these tactics to prevent parents from having time to pursue their right to an appeal of the decision.