My 7.5 month old DS can sit unaided for ages. Rolls over both ways all the time and has even started doing a little crawl type thing.
We used to hold him quite a lot, legs down, feet pushing against the floor and he liked it and was quite strong. But I did it the past two days and he sort of struggles with it, as if his legs buckled. This morning was a bit better, so not sure if he's just being lazy.
We have used the door bouncer thing since 6 months. Not massively, but maybe once a day, 20 minutes.
I used to find it very hard to sit my DS down - he would want to stand and so would be stiff as a board. Since he started crawling (about 6 weeks ago) I can never put him down standing, he wants to sit as he can crawl off from that position. Could that be it?
It may be worth a visit to your GP, tell him/her that your DC has previously had been "weight bearing" happily for some time using bouncer and if you held him, but suddenly does not want to.
He MAY have an injury to the hip or leg, or may have an infection, for example to the hip or knee joint, causing him pain.
While it is likely to be nothing, or something minor, suddenly not wanting/being able to do weight bearing when he previously could, should be a "red warning flag".......
(...... one of my DS had quite shallow hip joints and it was similar symptoms which alerted us to the fact he had developed hip pain as a result, at around 10 months, though his Hips had been tested as "normal" during routine baby checks.....)