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You may be expecting something out of him which he is not able nor ready to provide you with at this time (100% attention) If you truly are feeling insecure, that is something internal that you would want to work on (he alone will not be able to change in for you)
Him being civil with his ex and going to a family member is not inappropriate. He can still have some contact with people had had know for such a long time (20 years)
What you really would want him to do is to define some boundaries with his ex that you are comfortable with. Going to lunch for mother's day with her may be a bit odd. His separation is still new to him and you are doing well in exercising some caution in your own response to his behavior.