Welcome to the forum.
Sorry to hear about your son's story and his resulting diagnosis.
Please stop blaming yourself and bearing the brunt of this, you were in an emergency situation (saving fingers) and it's hard in that mindset to think of longterm consequences and ramifications of all involved.
I know it's tough, and UC is a tough disease to deal with.
Thankfully, you're smart to ask the right questions now, and see what you can now do to be proactive - and I'm sure you will be successful in that endeavor.
IMO, no it's never too late to start probiotics. But they take time to normalize the gut, especially after the antibiotics. I would recommend VSL #3 or better yet, have you doc prescribe VSL #3 DS ( 3 sachets per day) and it might be covered by a small copay.
VSL is especially formulated for UC, with 450 BILLION active cultures, and the DS with 900 Billion active cultures.
I would also recommend you look into certain supplements: slippery elm powder, L-glutamine amino acid powder, diet modification as in going gluten and dairy free, and LDN, low dose naltrexone.
However, now that your son has this disease, there are things
IMO that you can help him implement, which will have a direct impact on his disease, the
symptoms he will experience and their severity.
I was able to control my UC into remission through various
supplements, probiotics, diet modification, and LDN , low dose
For more info on the protocol which has put me into remission,
please go to this link: https://www.healingwell.com/community/default.aspx?f=38&m=1666318 , and look at the 11th post.
I hope you feel better and let us know how we can help
Rx meds didn't help me. But 4.5 mg LDN +Paleo diet +supplements=REMISSION.Protein shakes w/almond milk,Maca powder,L-Glutamine,D-Ribose,chia seeds.VSL#3,K2,VIt. D3,slippery elm bark powder.For more info,my protocol is the 11th post at this link: https://www.healingwell.com/community/default.aspx?f=38&m=1666318
If you have any questions, you can find my email address at my profile.