I have a similar situation. I just started sleep training our 10 month old son and we have a 3 year old DD who also will wake up with the crying. I am using the Sleepeasy Solution method and it has been SO much better than anything else I've tried or read, including the no-cry methods. They are sensitive and compassionate and address all sorts of issues including night weaning/feedings, co-sleeping, naps, etc. Personally, I think at 5 months they still might need/want a night feeding, but it depends on the baby.
Here's my suggestion...start sleep training at the end of the week (after the last day of preschool for the week), start with bedtime on night one, then naps and bedtime starting the next day. The book said it should take about 3 days to a week...but you should see improvement before your son has to go back to preschool the following week. I just did night 1 last night and am working on a nap right now. I was amazed last night...he only cried for 1 1/2 checks...I didn't even make it in for the 10 min check because he went to sleep (15 min total) and this morning, again 15 min (2 checks). Last night, for the first time EVER he slept from 8pm until 6:15am and no night wakings!!!!! He woke up rested and happy!!! I talked to my daughter before we started and explained that I need her help and we have to teach her brother how to sleep better. I let her stay up a little later (while I worked on getting him to sleep) and she loved the special treatment as the "important big sister". Then when he was asleep and it was time for her to go to bed, I explained that baby brother might wake up and cry for a couple nights, but that to help me and him, she needs to stay in her bed and try to go back to sleep, even if he wakes her up crying. She did GREAT and told me this morning that he didn't even cry! (She was listening) and she was so proud to have "helped him" sleep.
I hope it continues...fingers crossed!
Good luck with your efforts!