I found that Soy and Linseed bread keeps my both my girls regular. Sultana Bran or Allbran is excellent too if you want another option to the weetbix. I know what you mean about the formula. Maybe drop the number of formula bottles he's having and introduce a milk bottle instead? I'm no expert though. Just a suggestion. Bowel problems can be so stressful. Sending you big hugs xx