Broken promises

I am new to this website as my husband and I are just now becoming aware of his ADHD and its impact on our marriage.  We have been married for 20 years and have had our share of problems, leading us to marriage counseling at least 4 times with inconsistent results.  We now understand why.  Our son has been diagnosed with ADHD and as we are learning more about him we are making connections as to why we have many of the problems we have struggled with.  Among many of my issues is what I always called broken promises or him not being a man of his word.  I feel guilty about this now that I realize it is unintentional, but the consequences remain the same.  We have 3 kids, and I am noticing as they get older that they are frustrated with him often because he tells them he will do something for them or with them and then totally forgets about it.  This leads them to see him as unreliable and they get upset that he doesn't remember or just totally disregards what is important to them--keeping his word to them.  I am concerned about the impact this will have on his relationship with them as they get older (two of them are teenagers already) since I can relate to how they feel.  I don't feel like I can rely on him.  Any advice?