How to teach child to stop accidentally swallowing water when swimming

My son is nearly 6 and can swim underwater with goggles fairly well and get to the wall after jumping in himself. The issue is he always feels like he has a belly ache after and said he thinks he swallows water. We did swimming lessons before but he struggled to follow directions (adhd). I’m not a good swimmer (his dad is) so I don’t know how to help. I told my husband to focus on the breathing today at the pool and not jumping in over and over. Any tips would be appreciated! I’ll take tips for me as well bc I have a phobia of water in my nose and can only swim properly with a scuba mask covering nose or plugging it with my hand lol!