i would bet bigger on both turn and flop, we wanna extract + protect, so my odds would look a bit favorable tow a call, but still not sufficient if i knew he holds exactly 54.
vs 54 we have 22.7% equity on turn, pot odds there: 425 to call for 1198 = 35.5% -those we need for EV=0 call decision.
i agree that this opp never bluffs+ in this spot he has at least 2 pair always. lets count some handcombos: preflop it easy to give him every single 45o, 45s, all 33,66,67,6A.
some math:
say, he plays 90% of 45 hands this way, so lets say 14 out of all 16 combos.. fe w 5 :diamond:4 
he has board so crushed he might just call again and 1/15 of other times he might have raised on flop or he just calles; so vs 14handcombos we have 22.7%
there are 3 combos of both 33, 66 and 3 other of 67- so total of 9, i think he definitely plays this hands like this sometimes, u would have to have a ridiculously big sample to be sure of he is scared himself of 54 and doesnt play agro ... but-- fish wait for A to come so u hit and then they shove with set expeciting u to turn AK, isnt it so even with passive?
your equity vs 33, 66, 67 is 98.5%
The total minimal equity needed, as said: 35.5% hence:
14* 0.227 - x* 0.985 =0.355 ---> x =~ 3.1
so he just has to play 33, 66, 67 third of the time this way for u to make a good call here.. also: i personally think he might be more temted to slowplay 45 again, rather than slowplay his 2 pair/sets .. coz he might think now is the time to protect and get it in vs Ax
also: i didnt count A6, A3, A7 hands at all, and there might hop up 1 or 2 handcombos of those played this way too..
overall, i dont think its possible (good) to fold here, although, as said, u are behind in total prolly ~75%-85% of the time and its exactly the str8. Its just that the equity vs his remaining 15-25% is just monstrous, while even vs the str8 u are drawing to the moderate 10 outs, which makes it equityvise rly "good" -okeish
