Finicky about eating solids??

My little guy is 6 1/2 months and has been getting solids twice a day for about a month now. Some days he takes it really well and seems to love it and then some days he refuses everything and screams bloody murder when I try offering it. Does anyone's else's little ones do this???

I know if can take a while for them to get used to eating but I'm a bit concerned just because he has been considered underweight since birth and I would love to get the extra calories into him