Constipation is not usually a problem for the breastfed baby. It is not at all uncommon though for young babies to seem to have difficulty with a bowel movement, grunting and groaning as you describe. A change in diet can precipitate a change in stooling patterns. If your baby is otherwise happy and growing well, and does not have any of the traditional signs of constipation, such as dry and hard stools, streaks of blood in the stool, and a distended, uncomfortable belly, I would not be too concerned about his straining to have a bowel movement. In regard to your diet and your little one's gassiness, I don't think it is necessary for a nursing mom to arbitrarily restrict her diet.
Young babies are naturally gassy.
Most of the time one particular food that you eat, is not causing your baby's gassiness. If you think one food may be causing your baby difficulty, keep a food diary, noting not only the foods you eat, but the times of Ethan's gassiness. Then, see if the two are related. To help reduce your baby's gassiness, try nursing more frequently, so he will take in slightly less at each feed. If you are still at all concerned, it is very important to discuss this issue with your baby's Health Care Provider.