I had a Bourbon Red last year that didn't grow well. It kept up for maybe two weeks, then just quit growing. It remained active for another two or three weeks, eating and drinking. Then it lost energy and faded in about two days.
I had a turkey poult who was much smaller than the rest. At two weeks old, he weighed exactly half what his hatchmates weighed. He stayed robust, however. Feathered when they did, was alert and active. He just grew at a much slower pace. He is sixteen weeks old now, and he is still about 2/3 the size of his hatchmates. But he is the friendliest of the bunch. I bet he'll always be a bit of a runt.
Your poult will likely be the same way. Healthy, just small.