This is probably a really stupid question but I...

This is probably a really stupid question but I'll ask anyway. My 5 1/2 month old has been eating solids now for a month and a bit and I'm starting to introduce new things. Just lately (the last week or so) he screams out during bursts of farts. It seems like it's hurting him. It's also a little bit red on his bottom but doesn't seem like nappy rash. Just wondering if anyone knows if there are certain foods to avoid and if it could be the food causing this? He has tried most vegies, apple, banana and pear as well as lamb, chicken and beef. I try and make most of his meals from scratch but i have been buying ready made jars recently. Last night he woke up 4 times throughout the night because of the farting. Any suggestions would be much appreciated.