The ADA does not grant unlimited leave time. I don't see this as being necessarily an ADA issue, though FMLA is very much on point. While there might be ADA aspects to it, I think FMLA is a much stronger claim.
These are the points for which we need an answer, some of which have already been asked (but here they are all in one place).
1.) How long ago did this occur?
2.) How many employees did this employer have within 75 miles of your location?
3.) In the 12 months immediately preceding your firing, had you worked a minimum of 1,250 hours for this employer?
4.)In the 12 months immediately preceding your firing, how much medical leave had you taken no matter what the reason for it? This is leave both for yourself and to care for your son.

