My deepest sympathy to you and the children, for losing your husband so suddenly, and now having to cope with being the executor.
The fact that your late husband took into account the money already given to his son when making the bequests, makes it absolutely clear that he did not intend his son to have more. I assume your son has seen the will? If not it may help for him to see it if you are happy with that.
It was insensitive for him to ask at this time, apart from anything else, you may have 20 or 30 years more to live so why is he asking now? Could it be that he just wants to know where he stands so that he can plan his future? It seems that he has received a lot of help from you and your husband in the past and now he has to stand on his own two feet, it might be a shock for him to cope with after having a father to lean on. Perhaps he was assuming that the bank of dad was always going to be there and now he has to face the reality of standing on his own two feet (not before time). For example, if he is a freelance writer with an unreliable income, he may now have to think about changing his job and getting a regular salaried position. He may also be being influenced by someone else (wife or mother) ? Try not to think badly of him, he may just be a typical insensitive, slightly selfish man (especially if he is single - he has not learnt to be otherwise!). I agree that your reply was all that was needed and hope he accepts that and allows you to grieve in peace. Assuming that he does now drop the matter having just clarified where he stands, I hope that you and your daughters will still make him welcome as part of the family.
It sounds as though you have enough on your plate with having to sell a property and sort out the provisions in the will. I hope you can stray strong and once everything is sorted, you can take time to grieve and perhhaps, in time, pamper yourself and take a complete break away from it all.