limping behind is fine with 55
but 88 actually has good equity vs. his calling range

Hand 0: 57.733% { 88 }
Hand 1: 42.267% { JJ-22, AQs-A2s, K9s+, Q8s+, J8s+, T8s+, 97s+, 87s, 76s, 65s, 54s, 43s, AQo-A9o, K9o+, Q9o+, J9o+, T9o }

Hand 0: 51.071% { 55 }
Hand 1: 48.929% { JJ-22, AQs-A2s, K9s+, Q8s+, J8s+, T8s+, 97s+, 87s, 76s, 65s, 54s, 43s, AQo-A9o, K9o+, Q9o+, J9o+, T9o }

so 55 is a flip, and raising would only get him to fold some of his hands and get blinds out
but 88 is raising for value

a lot of fish will peel with middle pair type hands on the flop so 88 will be good more often on average if you show it down