simparica trio side effects: when should i worry?

hello all. would really appreciate some feedback on this. my 14 y/o cocker spaniel had his wellness visit and was given simparica trio. usually he’s been given bravecto, but our vet gave us this instead. he is now lethargic and not eating. he usually eats kibble but since he didn’t eat any of that, we gave him wet food. unfortunately he’s not really eating that either, just licking it. we’re going to boil some chicken for him and see if that will help. he has also had history of seizures but hasn’t had an episode in over a year. it would happen sporadically, maybe 4 or 5 times within his lifetime. overall, i am just wondering if anyone has had a similar experience? everything i’ve read about simparica so far is concerning me. wish i would’ve done my research beforehand.