I went through something similar with my (now 20 month old) son. He missed several physical milestones starting at 6 months - not bearing weight, not sitting up well without support.. At 9 months old, he refused to bear weight on one leg, and the other was not much better. He still wasn't crawling off his belly (he still crawls like a marine)and wasn't sitting up without support. The only difference is I was the one who was worried, and the doctor said he wasn't worried unless he wasn't pulling up by 15 months old...
I bought it when he wasn't bearing weight at 6 months, but I was terrified that - by 15 months, my son would be so far behind or have some kind of permanent problems because of something that we could have fixed early - so here's what I did, and here's what I learned.
1. I took him to a pediatric orthopedist at Cook's children's hospital in Ft. Worth. They did an x-ray to check for hip dysplasia (ball not in, or not firmly in, the socket), and to check leg length. His hips were in, and his leg bones were, within 1/8", the same length. (A difference this small isn't considered an issue, because of errors of measurement.)
2. I took him to a chiropractor - regularly. I saw her while I was pregnant (I had significant hip pain because of my own late treated dysplasia), for hips, back, sleep, and later to help the baby turn from breech. She would do adjustments on his spine and hips with a small metal spring loaded activator. For the next few days, he bore weight on both legs (and on another note, he only had a BM after adjustments - about 2 hours after... chronic constipation was a problem we treated with RX laxative until we started going to the chiro twice a week).
3. I had him evaluated at 13 months by ECI. Because he was not sitting up well, still belly crawled, was not pulling up - etc, he qualified for services from an OT.
She gave me exercises to do with him:
A. Lay him on his back, put one hand around his femur and the other holding his knee bent, and GENTLY push his femur into his hip. (Each leg, about 3 minutes a leg, a few times a day - like when I changed his diaper).
B. Stand him up and 'jump' him on the floor - feet first, to create impact in his hips and knees.
C. Keep him out of the exersaucer. Every professional I consulted said that putting them into an exersaucer or walker before they are pulling up to stand on their own actually delays the development of the muscles in their hips, legs, and abdomen that let them do it themself. I didn't have a Johnny Jump up (no door frames to support it), so I don't know if they're an issue or not.
Stuff your doctor could be looking for in an MRI - in order of likelihood.
1. Reassurance that nothing is wrong - it's just laziness.
2. Hip dysplasia - where the ball of the femur does not go completely into the cup of the pelvis.
3. Any kind of malformation of the legs or feet that would cause pain on trying to stand.
4. Irritation in the joints of his knees or hips - there are several (VERY RARE) issues that can resemble baby arthritis.
5. Even more rare - signs of osteogenesis imperfecta, which basically can result in lots of small (and big) fractures from everyday activities.
6. Some kind of nerve problem - which is probably already ruled out because of the actions you already describe (kicking, pushing horizontally).
For what it's worth, my son missed the 15 month pulling up marker - and the pediatrician started talking about testing, which I had already started 7 months before. Jason started sitting independently at 16 months, bearing weight at 17 months, and walking about 2 weeks ago. Now he's like a rocket. Maybe it's all the intervention - maybe it's just when he was ready to do it.
I know that BECAUSE I went undiagnosed for so long, I didn't walk until I was 3 and I still have almost chronic hip pain (which is vicious - it hurts to exercise, you gain weight, it hurts more). I participated in dance, cheerleading, gymnastics, and softball - but took Advil like it was candy from about 7.
Personally, I'd be glad my doctor is interested - have the test - and enjoy the peace of mind if it's just laziness... that way, if it's something more, you're well within the treatment window for just about anything - and your son will never know the difference when he wants to play soccer or football or WHATEVER at five years old (instead of still being in casts, pain, braces, and surgeries). :)