I had a sudden realisation the other night when I was hanging out with Beaker

that he hasn't gained any weight at all since we got him at 8 weeks old. He is now 5 months old. When the vet weighed him back in January he was 79g, now he is 80g. The heaviest he;s been at any point is 82g. I know this is just within normal weight range, but as he's still so young, shouldn't he be growing?!
We've been struggling to get him onto a pellet diet - we've found a brand that I've seen him eat, but it's only a pellet or two and then back to the seeds - and I'm wondering if I should just let him more seeds so he can get some weight on. Or take him to the vet.
Any ideas? Cheers in advance!