Do you have regular contact with this other family that didn't invite your son to either party? Do their kids and your son play together at all? If they don't, it's quite possible that this mom did not think to invite your son because you don't really know each other and little kids can have a stressful time having someone unfamilar at their party.
When you invited the children of this neighbor to your son's party, did they come? If they did then I'd say you have a reason to be slightly miffed that your son was excluded from theirs, however, if they didn't, I wouldn't worry about any of this at all. Even though your children are similar in ages, you can't force friendships (especially among aquaintences, if this is the case here) among children (and their parents) just because you live on the same street.
If you'd like to make friends with this neighbor, bring over a coffee cake and ask if you two can talk while your son and her kids have some playtime together. Or invite her over to your place for cake, conversation and playtime. If she's not interested, don't fret, she may just be too busy, too stressed or, to be blunt, someone you may be wasting your time on getting to know.
If your kids have played together before and do play together now, but your son was still excluded, be direct and let that mom know that you're disappointed that your son was excluded and were wondering if there was a specific reason why he was left out. Depending on her response you can take it from there.
D. L.