I've been where you are... and it's scary and upsetting and worrying and, and it's ok to be all of those things.
Stay away from Google ..always the worst imaginable problems, and see what the Paediatrician says. He/she may want to run a few tests, (basic blood tests, possibly an MRI) to rule out anything obvious. He/she will also want to do a pretty thorough history.. how was pregnancy, birth, any family history of late development etc.
After that it's probably a bit of wait and see. There are a LOT of problems that cause floppy muscle tone. A few are serious. The majority are not, and the majority if children make good progress in their own time. How is your little boy otherwise? Is he alert (so a purely physical delay) or are there concerns with his learning , social skills?
My DS2 was a floppy baby. Floppy from birth, didn't smile til he was one (or sit) didn't make eye contact either, didn't babble. Didn't do much at all to be honest. We had every test going, and I was heartbroken and worried sick.
16 years down the line if I could go back and give myself one piece of advice, it would be 'stop panicking, what will be will be, and whatever the outcome it will be ok. He will be ok and so will you' (Hindsight is a wonderful gift!) Because whether the issue is a mild low tone, improve over time one, or something more significant, worrying yourself sick won't change the outcome, but a calm clear head so you can ask the paed questions, push for tests, push for physio etc, is far more useful.
Incidentally my DS2 did turn out to have some long term problems, but he is walking, talking, delightfully loving and sometimes very typical teen now!!